As others have said, the idea of vassals being able to rebel is annoying but not the heart of the problem: the problem is a great king with high/absolute CA is basically powerless to stop his most petty underlings of underlings from creating chaos in his realm. "Powerless" isn't what we like to see when our empire is vast and we're doing an excellent job managing our direct vassals.
Honestly, there is nothing worse than seeing your hand-picked vassals get replaced by shitheads and knowing that you're only real recourse is to assassinate the shitheads before they win the war. Or, if like me your empire is largely full of same dynasty vassals, those "little wars" are never little. I've seen 150k stacks sitting on my vassals holdings. I don't even truly mind the in-fighting: it is being a powerful emperor/king with absolute authority that is powerless to stop petty lords from fighting over petty claims that annoys me. I should have more options than assassination.
I mean even if I didn't want to dirty my hands by actually picking a side, I should be able to influence that war half a dozen other ways. "My good friend and vassal Duke so and so - I'd 'hate' to see my dear cousin lose his duchy neighboring yours. Take this coin and help him squash that uppity count." Or, "Dear Cousin who is on the verge of losing his duchy: please see attached sum of money earmarked "BIG FUCKING MERCENARY FORCE." You should also be able to tell other vassals to stop helping so at the very least that small war doesn't erupt into half the vassals of my realm fighting it out to make Sir Shithead Duke of Worthlessland. Lastly, as another option, I should simply be able to join the war on either side at the risk to relations. It is the hard-coded way that we can do nothing but sit idle that is annoying. Nobody likes to be a powerless ruler.