Warning Wall of text incoming! tldr: I feel these guys are rather pointless atm, skip to the end for some suggestions. 

Unlike in WOM you can train these troops in ANY city not just the closest to the camp, a HUGE improvement. I also really like the mechanic that spawns the lower tier camps.

Not including the Serpents Pact you have the ability to train units from the following:

Darklings:

Empire Only. From Darkling Camp, original spawn point must have spawned a "Darkling Shaman's Army" otherwise the camp is cleared and looted by moving onto it.

Pros:

1. Can't honestly think of any.

Cons:

1. Requires Diplomatic Capital to recruit.  Unless you have found a Scenic View the building that produces this is several techs & hundreds of turns further on. 
2. No Customization, you are stuck with recruiting more of those crappy guys you were one-shooting a hundred turns ago.
3. Only Warriors, no Shaman or Warg-Riders.

Wildlings:

Kingdom only. From Wildling camp, same spawn method as Darkling camp.

Pros & Cons:

See Darklings, though they are marginal stronger.

Ogres:

Empire Only. Camp starts in play, I think, must be cleared before building.

Pros:

1. Uhh, no, can’t think of any.
2. Well, they do have brutal.  Good luck doing enough damage at attack 15 to have effect regularly.

Cons:

1. 9 hp.
2. Max unit size of 1
3. Requires Diplomatic Capital to recruit.  Same issue as Darklings.
4. 9hp
5. No Customization.
6. ALL of their stats are worse than the regular YOUNG ogres! (except move which is equal at 2)
7. 9HP!

Trolls:

Kingdom Only. Camp starts in play, I think, must be cleared before building though. Currently  bugged.

Pros: 

1. Regeneration could be handy I suppose. Though by the time you could get them how useful is 2hp a turn?

Cons:

1. Unknown, need to see the recruit able unit.

Dragons:

Both (Kingdom, Storm. Empire, Fell). Camp starts in play, must be cleared before building.

Pros:

1. It’s a Dragon!

Cons:

1. Incredibly rare? I’ve only ever seen ONE camp for these across all my games.
2. Game is usually long over by the time this would be researched.
3.  Limited Number matters as you may actually want to train these.
4. Max unit size of 1, lol.

Mercenaries: Both.  Bandit camps once a "Bandit Captain's Army" has spawned.

Pros & Cons:

Don’t know, the only time I have built the camps I got nothing new in the “Train” window and as they were part of the cities I couldn’t stand a unit on the camp to see if a new option appeared.

Scions:

Empire Only.  Recruitable after researching Scion Temple tech.

Pros:

1. Customisable like any normal unit.
2. Levels like a champion, looks like no access to the “path of the” traits though.

Cons:

1. Max unit size of 1

My suggestions:

• Change all camps to provide x no of [race] resource per turn. Say 0.25 for the Dark/Wildlings, 0.15 for the Ogres/Trolls & 0.1 for the Dragons. Same for the Serpents Pact allies.  Or have the camps provide a small amount of Diplomatic Capital
• Move the Dark/Wildlings tech to the 2^nd tier, between Knowledge & Agriculture, and have it require both Civics, Restoration & Exploration.  Re-cost to about 1.5 times tier 2 average cost.
• Move Ogre/Trolls to where Bonding Ceremony is now.
• Allow Ogre/Trolls to be recruited in “max unit rank -2” sizes. So if you can train regular guys in Companies of 9, Ogre/Trolls would be limited to one, three or five in a unit.
• Allow limited customisation of the units.  This could be done by making the first tab of unit design select the Race of the unit. This would then limit the weapons/armour and so on, depending on the race chosen.  I would say that none of the specials should be allowed magic gear.  Even the Serpents Pact beasts could be given a limited amount of traits to pick to represent training & perhaps special items to represent breeds.

For examples:
Darklings could be limited to the basic weapons & leather armour, only allowed Wargs as mounts & lock the Spell Resistance Trait so it cannot be removed. Add the “grants spell” traits to make a Shaman.
Ogres limited to one handed blunt & 2 handed (any) only, no mounts, armour or equipment, very limited normal traits. Add new traits for things like “Thick Skin” +1 def per lvl.

• And allow (Forrest)Drakes & Dragons to be bought as a mounts for Sovereigns only.
• On the subject of mounts once a Skath Pit has been built & Warg riding researched allow Skath mounts to be bought in the shop for Empire Champs.
• I actually think that all troops should level like the scions do so no suggestion for them!