S'okay, I start my war for independence while nearly every duke in the Byzzies is fighting to depose the Emperor. I can still buy them making peace to take care of me (I wouldn't want folks leaving the empire they were planning on taking over either) but c'mon, dozens of years of brutal civil war where the Byzzie army is reduced to nothing multiple times, and not a single outside enemy even tries attacking?
And jesus christ this emperor, 65+% warscore, I control the capital, they have no forces that could even remotely take me on - lolnope, it's just a flesh wound. At this point I started to run out of money (had two high-level merc companies to help me out) and ragequit. Moneylenders saved me for a bit, but one of the merc companies managed to disband by the time that event hit, and then a Byzzie army materialized out of nothing (this alone isn't so bad, I beat the previous ones quite handily) and invades my african holdings. Cue Emperor's willingness to fight going from 'it's just a flesh wound' to 'come on, you pansy!'.
So yeah. This game can be pretty frustrating. I hate how durable empires and kingdoms are. I've yet to see a single civil war or a war for independence to succeed, even if pretty much every single duke in the realm joins in.