i noticed one thing ( sorry if someone posted them already), if you eliminate a faction and got their capital, there's always some chance that a full stack of rebel with a 6 star general called, "blablah the Mauler" appear at these capitals ( and i got them alot) and siege the city, it's quite troublesome because your troops often are at border, when you come back, it's too late.
the only thing i know so far to solve this is letting these capitals governed by a family member with highest loyaty ( but they still pop out , of course, much less often), anyone know why they appear and how to avoid them ?