The absolute worst thing I can't stand right now is you can't join other people's wars to help them.
I had co-opted Leon as the King of Wales by getting my daughter to matrilinially marry the once removed heir, assasinated the heir and voila now my dynasty had two kingdoms.
Leon had actually inherited the kingdom of france along the way, but in a civil war with Aquitaine lost it. I'd done a lot of the reconquista at that point but couldn't goto their aid even tho I had nothing better to do while truces wore off with the muslims. So bitterly annoying.
My worst transition was a king who ruled for 50+ years, his son got killed in a battle, and the 3 year old grandson inherited. The spymaster regent (it always seems to be the spymaster) had terrible diplomacy and there went rebellions. Then when the grandson died his heir had so many bad traits every single one of the independent vassals tried to press their claims on the kingdom in turn. Every single lord died in jail. But hey, the King never got tyranny, nor did he lose a single region.