My time with 9th edition's been a little impacted by COVID and light surgery, but I've gotten my Custodes on the table enough to get an idea about how they feel.
Army Strengths
Durability
Your basic troopers are 2+/4++ guys with T5 and 3 wounds. Grab terminator armor for more wounds, and if you get a bike, you get T6 on top of that. You also shrug mortal wounds from psychic powers off on a 6+, which is nice. Most of your vehicles have either solid toughness, or high speed if not both.
ObSec
If you're a guy or a dreadnought, you hold objectives. Combine this with the aforementioned durability and you're good.
Accuracy
You're WS/BS 2+. Our HQs have re-roll 1's to hit auras, and at worst the enemy can inflict a -1 to hit debuff.
Melee Power
Outside of dedicated shooting units (IE - vehicles, Saggitarium custodes) most Custodes hit at a minimum of S6, AP-2 for 1d3 damage and they're swinging three times per model. If you feel like running a Telemon with a fist, it's S16 at a flat four damage, because sometimes you just have to make chunky salsa out of whatever's on-hand.
Deep Strike Access
You have a few flavors of terminators, and with From Golden Light They Come, you can deep strike other stuff (including Dreadnoughts) - and for a few more CP, your guys can roll 3d6 and drop the lowest on the deep-strike charge with the right host trait.
Liabilities
Army Size
You are a small, small army. At 2,000pts, a pure Custodes army might bring ~20ish guys. You have a small footprint. At least you didn't have to paint a ton of guys, right?
Psychic Defense
So you've got a 6+ shrug vs. mortal wounds? Great. You can spend a Warlord Trait (Impregnable Mind) on getting one Deny the Witch trait, and that's kind of it. You're looking at bringing outside help for this, be it Sisters of Silence or a Culexus.
Volume of Fire
You definitely have quality of fire with all the BS2+. However, there's a handful of units with actual volume of fire - Jetbikes pack Hurricane Bolters, and War of the Spider lets you bulk up Rapid Fire units (Aquilon Terminators, take note).
This tells you you're an assault army.
Random Melee Damage
Your infantry need a relic to have anything other than 1d3 damage on their melee weapons. Unless it's a 1-damage melee weapon. Your Dreadnoughts tend to get around this. Be aware of it when you're throwing down with multi-wound units - it's occasionally a little annoying, but bad dice can see primaris bobs and terminators soak more than one hit from high-quality weapons.
Point Cost
All your quality comes at a price - when you're building a list, your cheapest models are easily 50ish points a head. You may be filling in points with Misericordia, since that's about the only gap-filler.
Forge World
This is kind of neither here nor there - but almost half of your roster (and basically all of your vehicles) is from Forge World. Some folks still have weird prejudices against Forge World. You can build a decent army without ever touching resin, but your options are a bit slimmer.
On Objectives
Custodes take a couple of turns to get started. We don't have a lot of guns, and we don't have a lot of bodies. Pretty much all those bodies have Objective Secured, though - so you are well-suited to brawling over an objective marker. You also have a 1CP fight-twice stratagem for that.
When it comes to secondary objectives, your choices are going to be dictated by army composition - sometimes yours, sometimes theirs. I lean towards Attrition, Engage on All Fronts, and go from there.
Purge The Enemy
Assassinate is based on how many characters they've got - if you think they'll be near the front and you can tag them in assault, go for it. Titan Slayers is matchup-dependent - you can get a +1 to wound with Slayer of Nightmares for anything T6 or higher (T7 if bikes are fighting it). This really shines on terminators with either power gauntlets or castellan axes. S8 with +1 to wound is ugly. Slay the Warlord depends on whether or not you want to put the eggs in one basket.
No Mercy, No Respite
Attrition and While We Stand, We Fight are the standouts here. Custodes are hard to kill - and if you can get into assault, you should be able to get Attrition for 2-3 turns. WWS,WF is nice if you brought a Telemon and a beefy character or two - T5-6 characters with 7+ wounds and 2+/4+ saves are nasty, and a Telemon's got T8, 2+/4+, and 14 wounds.
First Strike is going to be difficult unless you have a very shooty setup vs. someone that left wimpy units out. You should probably skip it.
Battlefield Supremacy
Engage on All Fronts is doable with faster units - though you're probably only going to get 3 table quarters rather than all four. Linebreaker is going to be harder, I think, outside of a deep-strike heavy build, and even then it's potentially restrictive. Domination is another challenge for a smaller army.
Shadow Operations
You don't have a lot of units. Repair Teleport Homer might be worth it if you're planning on deep striking a support HQ and a squad into the enemy's lines on turn two - IF you think you'll kill enough in the drop, and you can let the HQ sit around. The problem is that actions are really for larger armies.
Warpcraft
Hey, you don't have psykers in your codex. Abhor the Witch is pretty much it, and is matchup-dependent.
A note on Force Org Charts when Building
If you're going Custodes-pure, give a look at a Patrol. Unless you're going to take three of a given slot, a patrol is realistically what you're after. You were probably going to leave a minimal troops slot on a home objective anyway.
Note also that Custodes have some awesome defensive stratagems - we can flat-out deny ALL re-rolls vs. a single unit, for example - so bringing large units lets us focus buffs.
Units of Note
The HQ Slot
Expect to burn 100 or ~200 points here. You can burn a relic to get a 3++ save if you don't feel like taking a storm shield (or if you take a terminator or biker). Trajann's worth a look because he brings a re-roll 1s to wound on top of the normal re-roll 1s to hit buff, but he's locked into a so-so warlord trait.
Pay attention to the Captain Commander stratagem out of War of the Spider - it's a nasty buff, and helps let you farm CP. Or pick from any of the other sick buffs.
The Vertus Praetors
Say hello to something you can build an army around. These guys are almost 100pts a pop, but you get speed (~14" move and FLY), durability (four T6 wounds), volume of fire (a hurricane bolter) and nasty melee (S6 spears with re-rolling wounds on the charge). It's not hard to see why folks build around these.
Aquilon Terminators
This is the forge world terminator. If you want to clear out infantry and then punch...well, anything that isn't flying...to death, these are the guys.
You land, you point your S5, AP-1 Rapid Fire 2 storm bolters at something within 12", and use Superior Fire Patterns to double your volume of fire. Eight shots per model is beefy. Then charge in and swing an S10 powerfist that hits on 2+. Things die if you can make it into assault.
If the enemy counters, use Auramite and Admantium, which means you ignore AP-1 and AP-2 - so you're either on your 2+ or your 4++. Given the cost of these guys, you need them to do as much work as possible.
Allarus Terminators
Here's your plastic terminator. You have fewer options than the Aquilon guys - choose between a Castellan Axe or a Sentinel Spear. Choose the axe for the strength; you can spend CP for more AP if that comes up, and if you need it you can get the bonus to wound.
Allarus terminators trade some shooting power (and two strength) for two neat stratagems - Inescapable Vengeance and Unleash the Lions. The former lets then target characters with their guns and grenade launchers. The latter lets them split into single-model units.
Sniping characters is a neat trick, but difficult - within 12", each Terminator kicks out two S4, AP-1 two damage shots and d3 S4, AP-3 one-damage shots. Given this is a 2CP stratagem, you really want like a 4-man squad shooting something more durable than a guard captain with an invulnerable save. Then you probably want to charge somethinge else.
Unleash the Lion is a neat trick more than anything. There are two things it does - first, it'll make the other side think long and hard about how much firepower they need to hand out, and where, as your terminators are still T5, four-wound models. Secondly, if you need models to split up and perform actions or claim objectives, it's an option.
Telemon Dreadnought
Don't be fooled by the S16, 4-damage fists - this thing is a wonderful gun platform. Grab a good position, stand still and unload with both guns and 10 shots from the bolt launcher.
When compared to the Caladius Grav-Tank, the Telemon is more expensive (by about 60 or so points) but much more durable - they both have 14 wounds, but the Telemon is T8 with a 2+/4++/6+++ vs. the Caladius' T7, 3+/5++. Caladius can out-range it with the right guns, and deducts 2 from enemy charge rolls. Also, y'know, it has FLY and a base move of 14", so can hide.
The Vexilla
You can take one of three standards on either a regular Guard, or an Allarus Terminator. One is a 5++ for friendly IMPERIAL units, so you can skip it for Custodes pure builds. There are two buffs to pick from - +1 attack per model, or -1 to hit vs. enemy shooting. If you aren't building a 100% mobile force, consider one of these.
Stratagems of Note
The base codex has some decent offensive stratagems, but War of the Spider introduces some choice defensive ones.
From Golden Light They Come
Deep strike 1-2 infantry, bikes or dreads that don't normally get it. Beefy at 1 or 3CP.
Golden Light of the Moiraides
If you picked the Dread Host as your shield host, pay 1-2CP for 1-3 deep striking units to roll 3d6 on the charge and drop the lowest dice. Suddenly those charges are a little easier to make.
Vexilla Teleport Homer
If your standard-bearer started on the table this turn, you can pay 3 CP to drop within 6" of him, and end up more than 3" away from enemy models. Is it a little niche? Yes. Are you going to nuke something close to you? Also yes.
Indomitable Guardians
Are you close to an objective, and would you like to fight twice for 1CP? We're an assaulty army.
The Emperor's Auspice
Pay 2CP, deny any/all re-rolls for a phase. Depending on the type of army you're facing, this is a nasty counter. You're incentivized to bring beefy units, since this is a flat 2CP to protect a unit for a phase. You will probably pay more CP for this than someone else pays to get those re-rolls, but if it stops something like Gravitic Amplification or Extermination Protocols, it was worth it.
Arcane Genetic Alchemy
Pay 2CP - wound rolls of 1-3 always fail. You're T5. Someone's high-powered weapons just got less effective. Use it when you expect to absorb more than small arms.
Auramite and Admantium
Pay 1CP - your terminator treat AP-1 and AP-2 weapons as AP0 weapons. I mean, AP-3 or better was going to hit your 4++ anyway. Honestly, if you have terminators, you should keep CP for this hidden away.
Superior Fire Patterns
If your infantry didn't advance, double your number of shots for pistol and rapid-fire weapons. Honestly, that's most infantry weapons. Aquilon Terminators and the Venatari Custodes are noteworthy (Aquilon have storm bolters; Venatari can carry S6, 2-shot 2-damage pistols with good range).
Tanglefoot Grenade
For 1CP, you deduct 1d6 from an enemy unit's move OR charge. It's a little niche, but you can potentially ham-string a unit twice if you're close to them.
Captain-Commander
Pay a CP, pick one of nine traits for a non-character shield captain. Strategic Mastermind is worthy of note as it'll let you reclaim one CP per battle round on a 5+ whenever you spend CP. The other buffs are geared towards mobility or melee upgrades, though there's also a flat +2W one. I guess if you wanted, you could dump a CP and relic onto a jetbike captain and have a 9W, 2+/3++ guy flying around the table shooting/stabbing everything he can to death.
Victor of the Blood Games
I mention it because it's a little iffy - on the one hand, it's undeniably efficient - it's 2CP, and it can a re-roll a turn, so you'll get more points back. You're putting it on a character, though, so you're already re-rolling the 1s to hit - so you'll be re-rolling wounds and saves, which aren't bad. But, it's still 2CP right off the bat.
Stooping Dive
Let's be real, this looks cool, but it's niche. You pay 3CP to let your biker unit charge at the end of the opponent's phase, and then they fight first. If the opponent doesn't know this is a thing, it's hilarious. If they fail a charge, well, then you gotta ask if you want to burn the CP to get in there. Or, y'know, you can move up, dump hurricane bolters on them, save 3CP, and then charge. It's an option, but expensive CP-wise.
On Shield Hosts
War of the Spider introduces shield hosts. These are analogous to legion traits, but in practice give you a warlord trait, a relic, and a bespoke stratagem.
The Dread Host
This is your choice for deep striking. Golden Light of the Moiraides (as mentioned above) is a nice buff for arriving from deep strike, since you almost always want to charge from it. As a bonus, these guys have a relic Castellan Axe that has a flat 3 damage in melee. The Warlord trait is an aura that says any 6s to hit within 6" grant additional hits, which isn't bad - but isn't super, since you aren't exactly a high-volume army.
Solar Watch
If you aren't deep-striking, this is a decent choice. The warlord trait is Sally Forth - if you start the movement phase within 1" of the warlord, you get an extra inch of movement, and can advance and rapid-fire with -1 to hit. You also get a neat little stratagem that's 0CP - if you kill a character, the next stratagem the opponent uses costs an extra CP, but you can use it once per round.
Overall
I've enjoyed Custodes thus far. They are rock-hard. The two biggest adjustments have been army size - you never quite get over feeling outnumbered, and the fact that turn one is almost never that graphic. That being said, I don't feel the biggest need to have turn one - I can't break the enemy's back on turn one with overwhelming firepower, and if I have anything in reserves for deep strike, I'm not sure I need to over-extend myself.
Plus, painting ~20 models and being done painting an army is pretty awesome.
