I'm newly the Wendish Emperor, as well as King of Poland and King of Lithuania. I also have enough of Pomerania to create that kingdom whenever I want.
Because I still have gavelkind succession (Poland's starting Crown Laws suck and I haven't had enough rulers yet to get high enough Crown Authority to change to Primogeniture), I'm contemplating giving away the King of Lithuania title and maybe creating and giving away the King of Pomerania title, so that I can choose how the Realm is divided up instead of having bits and pieces randomly redistributed on succession. (Right now a Polish county and the Kingdom of Lithuania would go to my second son, which would mess with the borders of the two kingdoms in ways I don't like.) I figure I can always remarry into those titles later anyway.
I've never played an Emperor though, so I'm unfamiliar with managing kings as vassals. I do already know that my Lithuanian dukes want the Kingdom and that has a hefty opinion penalty, but I don't know what sort of ambitions vassal kings have that might be worse. I've heard on various CKII wikis that kings as vassals are naturally "uppity", but I haven't seen any solid discussion of the mechanics that make them so.
I'm still on the fence about creating the Kingdom of Pomerania because I don't know enough to make an informed choice about structuring my Realm, and so long as I don't create the title my Pomeranian dukes and counts are happy as Baltic clams.
What are the advantages and disadvantages of giving away the King titles? Is it better to be emperor to kings or to dukes?