I'll clarify the situation I'm having. I'm Russia (started as Poland and have most of that myself) with Kiev, Novgorod, Denmark, Norway, Sweden, Saxony, Hungary and Sicily each as vassals (I gave the first two away to shut up a heap of whinging dukes, the rest were by claims so I've never held them). It's annoyingly easy to see 4 or 5 of them join a faction which generally puts it to at least 80% (each probably has 10-20% but there's now so many of them that it jumps up fast) and if I imprison some of them I may as well imprison all of them due to the collossal tyranny hit.
Course I can deal with them if they revolt (with a bit of difficulty, my personal armies are only ~50k) but with the frequency of it it simply isn't fun. I gave away Kiev/Novgorod to vassals the moment I formed Russia to stop it happening with the 20 or so dukes I had then as I'd spent probably 30 game years with ~8 of them revolting every 3 years or so and I was sick of it.
Course I have had games where it hasn't been a problem, but when it does become a problem it makes the game unfun in the same way the old rebellion whack-a-mole did and I honestly don't want to deal with it in a rediculously gamey way which the imprison/banish everyone method feels like because that isn't fun either. And before anyone says anything yes a large empire shouldn't be easy to hold together and I agree but if the game doesn't make it a fun experience then it fails badly.