|It's like this, but with power armor|
So, that being said - Reivers - let's take a look.
Squad Size - Start at 5, go up to 10, and they eat an elite slot. If you take ten, you can break into two five-man squads during deployment (or via stratagem).
Weapon Option - here's where you pick your role - each guy packs a bolt carbine (Assault 2 bolter, essentially) or a heavy bolt pistol (an AP-1 bolt pistol) and combat knife (good for an extra attack in melee).
Mobility - Now we're getting somewhere. You've got two options to pick from - grav chutes or grapnel launchers. If you take grav chutes, you can start in reserves and drop further than 9" away from the enemy. If you take grapnel launchers, you get two perks - first, you stop caring about vertical movement and second, you get to walk onto the enemy's backfield.
Assaulty perks - You've got a couple perks when it comes to assault - you have shock grenades, which disable overwatch and screw up enemy melee, and you also have the 'terror troop' special rule which forces a -1Ld penalty on anyone within 3" of reivers. So, y'know, being in melee with these guys is apparently scary.
Battlefield Role - Disruption
Reivers are designed to drop in and sow chaos - they aren't super-mobile with just a 6" walk, but you should be taking grapnel launchers or grav chutes so that you can get the drop on the other guy. From there, you leverage volume of attacks (either via bolt carbines or assault) and try to deal with either numbers of melee troopers or with shooty troops.
Shooty - you take bolt carbines. You drop in via grav chutes and dump bolter-fire onto soft targets from an annoying angle.
Stabby - you take heavy bolt pistols and combat blades. Pick grav chutes or grapnel launchers, and find the enemy's non-melee troops. Engage in butchery. PROFIT!
Comparison of Roles - shooty is guaranteed to get at least one good round of shooting in, and can easily grab good ground via grav chute. Stabby needs a good assault roll to start producing on turn one (you've got about a 28% chance of making a 9-inch charge) but can crank out a higher volume of attacks once you do get there.
NOTE - Black Templar chapter tactics with a re-roll to the charge give you about a 50/50 shot to make that charge, though.
Compared To Other Troops
Frankly, I don't see a point in trying to transport or walk Reivers - best bet is to leverage their deployment shenanigans. So, let's compare them to other troops that specialize in wonky deployment and screwing with the enemy's plans -
1) Assault Marines
You can drop in via jump pack assault, or can footslog it. Assault marines specialize in volume of attacks, though you gotta be close.
-cheaper per model
-more mobile per model (with jump pack)
-can take specialist melee kit (namely a fist or eviscerator on sarge)
-can take special weapons (flamers, or plasma pistols)
-fewer attacks per model
-less durable (one wound per model vs. two)
-must get into melee to do damage
Both units specialize in volume of attacks - assault marines (and stabby reivers) need to get in close, and the carbine rievers do it from range. Rievers are more durable and more killy per model, but also more expensive. The big perk assault marines have is specialist kit - flamers or a higher-strength melee weapon.
2) Drop Pod marines
I'm listing this because of the option to take grav chutes and bolt carbines - which means you set up where you want and dump bullets into people.
-more options (heavy/special/combi for tac marines, special issue bolters/combis for sternguard)
-drop pod can take up space (a la objectives, deny deployment)
-ability to bring more than just boltguns
-significantly more costly - a drop pod pretty much costs as much as a minimal reiver squad
-you have to keep more units in 'reserves' - drop pod + squad versus a squad of rievers
-Rievers don't have to get within 12" to unload two bolt shots
-rievers are more durable per model
-rievers are more able to defend themselves in melee (admittedly sternguard have 2 swings per guy)
Honestly, drop pods are kinda 'meh' in 8th at this point - I lean towards Reivers here.
I'm throwing out scouts here because they can fill similar roles. Specifically, I'm talking about bolter scouts or pistol/CCW scouts. Sniper scouts are entirely different.
-fill up troops slots (for things like a Battalion force org)
-deploy normally, but outside of the DZ - so you can screen off deep-strikers
-can bring a heavy weapon
-can bring a specialist melee weapon on sarge
-less durable (single wound, 4+ save)
-deploy on table - so the other guy DOES know where they are
-less damage output (less attacks per model)
Really, the big thing scouts get over Reivers is that they're troops (if you're looking to fill out a battalion force org) and that you can use scouts to screw over someone else's choices that drop from reserves.
Notable Chapter Tactics
Ultramarines - More applicable to shooty reivers, since they might want to fall back instead of fight.
White Scars - being able to charge after falling back could help stabby reivers if the enemy threw a screening unit at you, but you could still get into melee with something important.
Imperial Fists - cover-ignoring bolters are fun, period.
Black Templars - Stabby Reivers are totally down for re-rolling charges, ESPECIALLY the 50/50 charge chance.
Raven Guard - shooty reivers love this - get those two shots from 24" away, and make the other guy either get closer or take penalties to hit.
Iron Hands - more wounds per guy mean more chances to take that 6+++ save and laugh.
I think the natural partner for Reivers is actually the Terminator Assault Squad. Why? Deep-striking assault troopers really, really don't like cheap screening units (cultists, scouts, that kind of thing). What kills screens? Boltguns. What do reivers have? Boltguns.
They also go neatly into a Vanguard Detachment - your assault terminators probably want a chaplain (especially if they have thunder hammers) and then you just need one more elites choice to fill out a small detachment.
And even on their own, it's nice to be able to dump a handful of durable guys on an objective, especially if it's in cover.