Yeah, taking over vassals from realms with weak rulers (like the ERE) is a sure way to end up with extremely bad contracts. No taxes, council rights etc., it's usually best to provoke as many uprisings as you can and then revoke all those titles, only to give them out again. That's the only way to reset those contracts.
It's best to stick to republics, far better conditions, and their succession doesn't create countless sub-vassals (to whom your own crown authority doesn't apply).
Edit: Well,
@prismaticmarcus, since I only stated objective facts and you disagree with one or more of them, would you be so kind and explain a bit with what you disagree with in particular? I'm interested in hearing you out on it.
Edit2: The same for you,
@YellowPress, don't just hit disagree. Tell me what you're disagreeing with, as this is a forum and I'm interested in hearing and discussing opinions. Nothing I said isn't objectively true. Going around and hitting disagree for the sake of disagreeing is just bad style.