cultural conversions are bugged in the previous version - instead of an MTTH of 1/2 millenia (doable but not too rapid like vanilla), it'll take 12 millenia in that version
as for opinion of vassals, part of the reason is that you're not supposed to be able to comfortably hold a large empire. but then again, it's not exactly good balance either to just rely on opinion to make large empires unstable - factions should also have a larger impact at that point (which I haven't exactly gotten to balancing yet).
I'll still need to fine tune the balance of that, but not any time soon though. For now, it'll stay that way temporarily until I could work on it.