Mephrit - +3" range / +1AP in half range - AKA "assault/rapid-fire love"
Mephrit is pretty straightforward and usable.
If you were on the internet, you saw far too many times where someone asked "flayers or reapers?" on warriors. Mephrit gets reapers to 15", which is a lot more usable with transports and the Veil of Darkness. Otherwise, the basic trait is great for making your small arms a bit more killy. Your preferred protocol is great for your rapid-fire turn, as it strips cover AND might give you more AP if you roll a 6 to wound. Note that another 3" of rapid-fire range usually doesn't mean that much, but assault weapons - IE Tesla - tend to like it.
The unique stratagem just helps more - you can get mortal wounds on 6s to wound, up to a max of 3. Mortal wounds are always great, and most stuff with the volume of fire for this has AP-1 (maybe AP-2 if you're rapid-firing Flayers, because of the +1AP bonus.)
The relic is not BAD, per se, but you probably won't take it because you've got other options. Your Royal Warden (...if you took one) gets a 36", rapid-fire 3 S6, AP-2 2 damage gun. Again, this isn't BAD at all, but you've got some solid relics to pick from before this.
The Warlord trait is, interestingly enough, more stabby - +1A, +1S. Compare/contrast to Enduring Will; this is a bit more killy but Enduring Will is a boon against multi-damage weapons. Note that +1S gets most Lord weapons to S8, which is a significant break-point for your wound rolls.
Novokh - +1" to charges / +1 attack on the charge - AKA "CHAAAAAAAAARGE"
Like Mephrit, this is pretty+ straightforward and usable.
Do you want to assault things? This is how you assault things. A Lord with a Veil of Darkness and Implacable Conquerer next to some Lychguard and get ready to have some fun - you can do silly things like use the standard +1 attack stratagem for Lychguard, AND Novokh's +1 attack stratagem, AND +1S for CORE melee units. Going to an 8 with a re-roll means you get about a 65% chance to make that charge. Not guaranteed, but damned scary since it might be game-changing.
The exact bonus of a +1 to charge rolls depends on where you're at on distance - could be as much as ~17% (8 to 7) , could be as little as ~6% (12 to 11).
Your chosen protocol is decent - +1S and bonus AP on 6s to wound. +1S matters mostly if it'll kick you from wounding on 4+ to 3+.
Your warlord trait is so-so - you get free mortal wounds on a 6 to wound. I'd rather get more attacks or better attacks.
Now, the relic is pretty straightforward - it's good for +2 bonus attacks. Again, it's straightforward, and useful.
Szarekhan - 5+ shrug vs. mortal wounds / re-roll a wound - AKA "Utility and psyker defense"
This is some straightforward stuff as well - you aren't necessarily pointed towards any given build, but your units with low-volume / high-quality attacks are going to like it. This is by and large the most utilitarian dynasty, as you also get a stratagem that gives you a Deny the witch on a flat 4+.
Your utility extends to the Warlord trait - it lets you use one protocol twice. The problem remains in guessing the flow of the game well enough to pick a protocol - so I'd recommend you pick one to double up on in the first couple of turns.
Surprise, surprise - your relic, the Sovereign Coronal, is ALSO utility - based. Your command protocols are 9", and any CORE units within that range get both sides of the protocol. So, yes, there's utility here - however, you need to REALLY get your protocols right. I'd honestly skip this one.
I want to like Szarekhan, but the most reliable benefit is going to be the wound re-roll, followed by the mortal wound shrug and the psychic deny.
Nephrekh - Translocation / 6++ invuln - AKA "speed kills"
Nephrekh's 'translocation' lets you get an auto 6" advance in exchange for no shooting w/ assault weapons, AND it lets you fall back through models. You'll probably use the auto-advance more than anything else. Your mobility extends to your dynasty stratagem - you can slap a DYNASTY unit in deep strike (...sadly, no C'tan).
Your warlord trait and relic help you leverage this mobility into assault fun. Anyone taking a shot at your warlord takes -1 to hit - this helps more when folks aren't swinging powerfists, etc. at 'im, but it's still very usable. If nothing else, you get access to a pair of defense WL traits in this and in Enduring Will. Your relic - the Solar Staff - is a Staff of Light with a better rate of fire - but the real perk is that if you land one of those six shots on target, you turn off Overwatch and Set To Defend on your target.
All in all, Nephrekh offers some obvious utility - it's beginner-friendly with Translocate, and anyone can make use of the speed and the Deep Strike stratagem. The 6+ invulnerable save is neat - won't save you all the time, but might save you here and there. Scarabs kind of like it, to be honest.
Nihilakh - Ignore AP-1 in your Deployment Zone / Obsec Enhancement - AKA "go read custom dynasties"
9th is game of objectives. Objective secured is always going to be useful. Your most noticeable trait is that EVERYONE is now objective secured, and models that already have it count as +1 model for the trait. Ignoring AP-1 in your DZ is kind of 'eh' at best - it's not bad, per se, but it's not super. That's ok, ObSec matters.
Your stratagem is 'eh' - you pay a CP to let infantry be able to shoot. It's not great, but it can be useful if you just need a little more shooting. In general, though, you don't want large, important units doing actions - but this is a nice backup plan.
Your warlord trait is just 'fight first' - not bad, but might save you if you don't nuke your target on the charge. Your relic is similarly 'eh' - it's +1 to your save and ignoring wounds on 6+. Going to a 2+ armor save isn't bad, but you've got better options, frankly.
I want to like Nhilakh, but frankly I think we've got better options - you'll see when we get to the custom section. The real money-maker here is that ObSec frees up how you can build, and lets your faster units hold ground against non-Troops infiltrators.
Sautekh - Morale re-rolls / 18" rapid fire - AKA "Y'all like Gauss, right?"
You're here for the second one. It's not hard to build around rapid fire, given that pretty much anything that can take a can can probably take rapid-fire. Your stratagem piles on with the love, giving fellow Dynasty units a +1 to hit if someone just SHOT at it. Honestly it's kinda gruesome if you just want to delete an important unit.
Your warlord trait is pretty utilitarian - you refund command protocols on a 5+. Your relic is utilitarian and defensive - your warlord makes an enemy unit within 3" fight last - actually better than that, they can't fight until everyone else does. It's neat if you get charged, OR if you want to get the drop on a nasty melee unit.
Overall, these guys aren't terrible.
You get to pick from one of eleven Dynastic Traditions (I'm skipping the 'we use another dynastic code') and one of seven Circumstances of Awakening. You don't get unique warlord traits, relics, or stratagems. I'm not going to touch every last one of these in depth, as some of these are partial re-treads of other Dynasty Traits.
Dynastic Traditions - Re-Treads
You can get Objective Secured (Nihilakh), Re-roll a Wound in shooting (Szarekhan), 6+ invuln (Nephrekh), and Morale re-rolls (Sautekh). Really, you care about these when it comes to mixing them with Circumstances of Awakening, as it's that vs. the dynasty that includes this already. Mixing Nihilakh's Objective Secured (or double Obsec with stuff that already has Obsec) is the real sexiness here.
Dynastic Traditions worth Looking At
Rad-wreathed is worth a look for assault armies - it's a 1" aura of -1 toughness. S3/4 and S6 melee units care about this. It's worth noting that, outside of a couple specific stratagems, this really helps Flayed Ones - they start wounding T4 infantry on a 3+, and T5 infantry on a 4+ - which REALLY matters with a volume of attacks.
Pitiless Hunters looks neat - it lets rapid-fire weapons double-tap out to max range. Honestly, though, if you're really looking to buff rapid fire you pick up Sautekh or Mephrit and get their cool stuff, unless you just want to try it with a Circumstance of Awakening.
Immovable Phalanx - if your INFANTRY stand still, they get +1 to save vs. 1-damage weapons. Honestly, you'll use this on objective campers and stuff that's already stuck in assault. Sounds cool, sounds fluffy - it'll kick in a couple times a game.
Contemptuous of the Codes - +1 to hit vs. Characters. Um, neat, a to-hit bonus vs. one to three or four units. I guess if you REALLY want your snipers to hit, this is a thing.
Severed - your command protocols work within 9" of characters instead of 6". Protocols are neat and all, but seriously, it's a case of opportunity cost here. Protocols are neat, but you're doing well if you managed to pick relevant ones for the first couple of turns.
Circumstances of Awakening
This is where it gets neat - you've only got one re-tread here, as you can get +3" range (Mephrit called, asked what's up.)
If you really want to go Canoptek-heavy, you look at The Ancients Stir - it's a +1 to move for Canopteks, and a 6" pile-in. This is neat and fluffy more than anything, but faster Wraiths and Scarabs should scare anyone.
You can take Arise Against the Interlopers if you want to auto-wound INFANTRY and BIKERS on a 6+. I mean, it doesn't sound bad but I really have to ask if it's worth giving up your regular stuff. High rate of fire troops probably aren't going to worry about wounding infantry THAT much. If you're worried about wounding T5 or T6 infantry/bikers, you probably also want more AP than wounds.
Healthy Paranoia is a re-tread of Mephrit's +3 range, so realistically if you want to run Reaper warriors or Tesla Immortals/Tomb Blades, ask if you want to mix it with another Dynastic tradition or take Mephrit.
Relentlessly Expansionist is the one everyone is (rightfully) raving about - move 6" at the top of the first battle round, but before the first turn starts. This is so much utility it just ain't funny, especially since you can get out of your DZ.
Isolationist looks interesting, as it's +1S for rapid fire shots within 12". Going up to S5 on Gauss Flayers is neat (S4 to 5 matters vs. most infantry) but again, I feel like Mephrit or Sautekh might give you more.
Warrior Nobles is lol-tastic - your Nobles (all one or two of them) re-roll 1s to hit and wound in melee. Um, sure, go ahead and take it, but do it for fluff reasons.
Interplanetary Invaders is an interesting one - your vehicles can either fall back and shoot with a -1, or shoot in melee without a -1 on heavy weapons. Honestly, the fall-back-and-shoot is cooler, but vehicle-heavy necrons seems...um. I dunno. I'm not feeling it.
Eternal Conquerors + Relentlessly Expansionist - Obsec on everything and a free 6" move before the first turn is crazy useful. It's just stupid useful, to be honest - literally any build can make a good use of this. Move up, grab and hold objectives. This also lets you deploy more aggressively, and then either exploit that OR adjust.
Rad-Wreathed + The Ancients Stir - Scarabs and Flayed Ones get the most out of this. Scarabs are stupid useful, and with this they're faster and wound most infantry on a 3+ or 4+, and they throw a LOT of dice. Flayed Ones will love it as well.
The Necrons have some solid options when it comes to picking their Dynasty. If you want to go hyper-aggressive with assault, Novokh is your code. If you like short-range firefights, Mephrit and Sautekh are worth a look. The remaining dynasties have varying flavors of utility, but I lean towards Nephrekh for the speed and the deep strike.
The custom dynasties are full of neat, fluffy options - but you also have some crazy stuff, like army-wide obsec and a free 6" move off the bat with Eternal Conquerors + Relentlessly Expansionist.