Personally my only complaint about them is the way they interact with the republics. Republics have to dish out the amounts that monarchies do PLUS around 15-20% to the traders too. This makes the already suffering republics suffer even more from having their lands crippled.Cossacks is not a bad DLC. It adds pretty cool stuff and estates that are ...arguable. I think they are fine as they are, but could be way better. Meiou and Taxes shows us a completely different approach to estates and one that players expected. I just want to say that estates are not badly done, but they could be done way better.
That and frigging horde tribes. I swear that every time I'm a horde and I hit the reform button, it is while cackling maniacally and going "SO LONG YOU ABOMINATIONS I WILL SEE YOU ALL IN HELL" as my tribes vanish.
It makes the annoyance of then having to dish out the lands to the clergy and nobles just somewhat less agonising knowing that finally I'm free of the tribe-shaped shackles. The tribe estate really needs a lot more bonuses for what they do, since they've been balanced around player hordes in multiplayer. These are people using them to their max.
Compare them to the AI or casual players (like me) using hordes and the tribe estate is just a money-guzzling monster.