Concerning Crusades: Currently running an Crusaders XL high campaign and have found more reason to go against my fellow Catholics than any other factions.
I've been turtling the Crusaders, only going after ex-commed Cats after the Pope asks for help and dropping allies that attack fellow cats/are excommed.
Not surprisingly, the Pope asked for help with the Sicilians and Venetians after getting the worst of it in wars against them. I was able to take Sicily, Malta, Naples and Greece in this way. Next up, the excommed Genoese and Spanish, though I have finally begun crusading against the Horde.
I've been at peace with the Egyptians from go (though not allied), as they were much stronger than me when the Pope asked for a Crusade against them. Several Catholic crusades have failed against them.