I noticed that the units file for the Undead faction, when downloaded from Steam, had an "a" as first letter before it's proper name. It's possible that that error caused another faction to be used, a bit like what happens when you use mods that add other factions and also use CoS, without full support for those other mods. If other references go to its proper name, then the error would prevent normal working of the DLC as it would not find the Undead units. I renamed before I started and my undead towns properly use undead and the undead rules.
Undead cities don't have any natural growth, but any growth bonuses still work, up to the Grave Limit. So this means Tower of Dominion (+1) and the Forgetting special building (+3) work, and even Sovereign's Call (+2 Growth) should work if you happen to get a champion with Life Magic. There is a special city spell that gives any city that has it +1 Growth for each Death Shard that you have in your empire, but I'm not sure if you actually have to have it built - I had a Death Shard and it got razed by wandering monsters, and when it was rebuilding I still got the bonus. Maybe it has that old bug that prevented other spells from updating the next turn, which has been fixed for all the core spells.
Btw: can Stardock, can you remove Kingdom as an option for the Undead faction? The way it is portrayed in the game, as well as the way it is normally portrayed, prohibits Undead from ever being a "Good" faction (except in Age of Wonders 3 hehe). And the Morrigan spell is useless if you are Kingdom. Basically, in this game, Undead are Evil.