The default mid-game crisis, the Great Khan, has incompatibility with Ascention. Basically, if you ascend while you are vassalized by the Khan, it bugs out and the diplomacy with the Horde is broken. You can still destroy the khan, but you have to do it manually by attacking each individual thing belonging to the Khan. And if the Khan managed to get strong enough to vassalize the player, that is alot of stuff to destroy.