If you know how to set up a stable realm in Conclave, it really doesn't matter if you're an Empire or a Kingdom until you get large enough to control all of Europe.
Basically, they set up vassals to basically be insane, with a -40 opinion malus if they aren't on your council, only a +15 opinion bonus for being content, and only a -50 malus for killing their parent, so that basically, murdering someone's father in cold blood makes them only *slightly* more angry than if you don't put them on your council. Basically, they're all insane.
To "balance out" the new mass-insanity of vassals, they added the non-aggression pact upon marriage or betrothal mechanic.
So, now if you know what you're doing, you simply won't have any revolts, ever, unless you get unlucky several times in a row.
I'm currently playing a game (as Muslim) from a start as the High Chieftess of Kiev, and 400 years in, I control Ruthenia, Poland, Lithuania, Rus, Finland, Crimea, and half of Perm. That's a realm size around 300, with a capital at Kiev, a personal demesne of 10, and with vassal levies cranked up to almost the top. My current ruler's grandfather created the Empire of Russia title.
I have seven (7) vassals above baron level. Six of them are Dukes, and one was a Duke who just got given the Viceroyalty of Finland, so I guess he's sort of a King now.
Basically, you set up the guys who you want to be dukes with a good base demesne of their own, so they can easily smash any revolts from beneath them, and give them four or five duchy titles each. this gives them enough personal troops to smash revolts, and enough levy troops to effectively play whack-a-mole with the near-constant Viking raids without your involvement.
Then you pump out babies like mad, and set up a few of your sons as bonus baby farmers, and marry into every duke's family. Basically, you want the current Duke AND his heir married into your family, with enough spare children (or adults) floating around so you could do it again TWICE on very short notice. One trick is what I call "the double betrothal." You betrothe a newborn to some 65-year-old, which cements an alliance for x number of years. He'll die long before the newborn is marriage age, so when he dies, you betrothe her again, to his son or grandson, and the alliance continues.
Now you're revolt-proof. You can literally arrest and execute half the realm, get 250 tyranny, have people warning their children that you'll show up and eat them if they ignore their vegetables, and you're still revolt-proof. The downside is that your income drops sharply if everyone despises you, so don't get too kill-crazy, or you'll be too poor to keep your personal troops mobilized in the field for too long.
Basically, it looks like they wanted to make blobbing more difficult with the Conclave changes, but basically they made it much, much easier, as long as you change your play style to fit the new game mechanics of "stark-raving mad insane vassals who'll murder for a chair on the un-empowered council, but who can't attack in-laws at all for any reason."
Now, this same play style is available to pretty much anyone. Muslims with multiple wives, and Dharmic-Religion and Pagan folks with concubinage have it the easiest, but even Christians can pump out enough kids if they pick lustfull wives, say "yes" to the girls in the woods if they have hunting focus, or simply take seduction focus for five years or so to get 3 or 4 lovers... except for the fact that Christians are often stuck with Gavelkind, which destroys realms.
TLDR: the reason Muslim realms appear more stable now is that a Muslim ruler with 20 kids and Open Succession thanks his lucky stars that he has so many kids to secure alliances with, while a Christian ruler with 20 kids and Gavelkind succession curses his "bad luck" at having 20 kids who will rip his realm to shreds.
The root cause is Gavelkind leads to weak realms and Open Succession leads to powerful realms, which is 100% historical.
