Slitherine

When playing as chaos in a skirmish campaign, units can gain experience but there are no choices to make that I can see when it does level. In the next match, if I do get the unit again, it will show as one level higher, but I have yet to see any new abilites or changes to offensive or defensive stats.

In addition to not gaining new things, not being able to pick an ability also means the loss of the sudden restoration of hit points in the middle of a match that comes along with leveling.

In contrast, space marines, imperial guard and orcs leveling works fine, I get the choice of 2 abilities on level up and regain some hit points on selecting one, so I was wondering what leveling does for chaos units?

I heard they gain additional health. Can you check?

Yes, just need to progress a little in this campaign to get some levelled units. So far I have a level 2 Herald of Khorne

Herald of Khorne, level 2
Hit points 600, Ballistic Armor 100, Heavy Armor 60
Move Points 34, Actions 2, reactions 1, Morale 220
Weapon: Hellblade - range 1, accuracy 80, shots 1, damage 80, power 100, piercing 90, shock 8
Abilities: Murderous Tide - 15 hp and 10 morale per kill

I don't want to end the current game, so I can't compare to a level 1 Herald of Khorne just yet, I'll update in a couple of hours .
I hope it does something or later will do something, levelling is quite fun. I had a game with a level 4 Venerable Dreadnought with at least 2 gun upgrades and went on to destroy a Lord of Skulls over the course of 3 rounds with it's heavy bolter. At the start of the campaign, I thought more of it's gun as a pea shooter than anything dangerous hehe. That dreadnought truly had the wisdom and and combat experience of countless battles.

Update
Ok, here is the stats for a level 1 Herald of Khorne

400 Hit points, Ballistic Armor 100, Heavy Armor 60
Move Points 34, Actions 2, reactions 1, Morale 220
Weapon: Hellblade - range 1, accuracy 80, shots 1, damage 80, power 100, piercing 90, shock 8
Abilities: Murderous Tide - 15 hp and 10 morale per kill

You are right Parallax, it does add hit points, although nothing else on that unit.
As long as it does something I'm happy, although a little more variation, if it is only hit points for every unit, would be nicer. It is nice to know, really, that one rummaged in the back of my head, thanks for pointing me in the right direction .

Aha, I found part of the confusion, on the army point buy screen, the difference in hit points between a level 1 and 2 unit are not shown, although upon inspection on the subsequent deploy mode, a levelled unit will show a higher hit point total than before.

Edit: and then it shows fine the next match, I don't know. Anyway, it was solved, all is good.

Edit 2, a bug in the point buy screen: There is a bug with stats at the army point buy screen, the match it showed too low hp, it showed level 1 hp for all units of that type, ie all Demonettes had the same hp on the screen, irregardless of level. The next game, it showed the stats for level 2 demonettes on the screen and this time it was correct. The next match however, I noticed a lot of level 1 units with the same hit points as level 2 units, however in game the level 1 units had less hit points. It seemed the screen had updated, but now did not show correct hit points for level 1 units, instead showing the hit points a level 2 unit of its type had (as I verified while in game and the actual hit points were shown).

Second bug (I think, pretty sure though) demonettes do not appear to level: I'm playing with the Bedlam storm army of chaos demons and thus have only 2 Demonette squads, iron man mode is on as well. 1 Demonette squad got to level 2 already at the first match. In 2nd match, they showed as level 2 on the point buy screen, but in game was still a level 1 unit with the xp bar saying "levelled up" or some such. This has persisted on 2 the 3rd and 4th match (I've guarded them well, they are neat and it seems they are not able to level properly.

This is happening with chaos spawns as well, I got two of them maxed in xp and they showed as level 2 after when I bought them, but in the actual match it entered as a level 1 with maxed out xp. Other units seem to level fine, such as the Herald of Tzeentch, Herald of Khorne, Flamers of Tzeentch and so on level just fine. Most units seem to level fine, but so far the Chaos Spawns and Demonettes do not.