I've got a game right now as the empire of Carpathia, starting from the 867 start. As everyone knows, yes the Magyars are conquest beasts still, especially if you stay Tengrii. My empire stretches from the Pyrenees to the eastern edge of the map. There is no way I could deal with that many dukes or counts, so I've formed every single kingdom I can once I get all the de jure territory. It's an elective empire, and since I formed both the empire and every kingdom after Hungary after reforming the faith and switching to elective, all the kingdoms are elective too, which I *think* helps keep them calm. I have the occasional issue with a king that just inherited, but only for a little while, because I always train my heirs (plus spares thanks to elective) for diplomacy. I have a slight malus due to being king of Greece as well as the emperor, but otherwise everyone loves me. And at last count I have over 20 king vassals.
Of course, it really helps to make sure to give away all holdings to people of your religion and culture, which is easier for some than others. It's dead-easy for Tengrii thanks to conquest and tribal invasion CBs, but my Norse ruler of the kingdom of Ruthenia in another game that used his once/lifetime subjugation to gain the entire kingdom of Rus got stuck with a lot of wrong culture/religion guys he couldn't get rid of easily. But still, high diplomacy (and later high prestige) rulers can usually keep everyone in line. Especially once you start getting enough retinue that your retinue alone is larger than any individual vassal's levies. Which is why it's probably a good idea to start spamming skirmisher (or longbow if English/Welsh) retinue until you have a large enough cap to replace them with 10 - 15k of something else (unless you are English/Welsh, then just keep them). Faction strength and how dangerous a faction is depends a lot on the combined numbers of your troops, not the actual strength of the troops. Cheaper retinue cap troops = more bodies = less faction strength.
