Only game I quit (out of two, considering I just got the game 3 weeks ago, both in Ireland) was when I figured I was screwed. Made Ireland from Dublin, but it wasn't a unified Ireland--Scotland, which was highly unified and had I think high CA, took Connacht very early when he got himself excommunicated, then Breifne, then claimed Kildare, then usurped the Duchy of Meath, then kicked my ass to take Dublin. I was able to win it back by striking when they were on the Crusades and dealing with a pagan Moray (and using mercenaries), but I quit when it was clear my game was just about done. This came about right after Scotland had joined the Duke of Ulster in his plot to lower crown authority, and once my levies were utterly ruined and I was back at autonomous vassals, he formally declared war to take back Meath (with no truce--maybe there's no truce if you join a war but aren't the primary parties to the war?).
I gave up at that point (in the 1180's)--I would be king of Ireland of 4 counties in the south, a vassal Duke of Ulster that would probably just as soon go independent or pick a stupid fight with someone and lose, and with me in no position to resist the inevitable Scottish pressed claim on any of my other counties that would lead to probably losing Leinster, rinse, repeat...