The only bugs I have come across that spoils the game, are when you leave abberfraw (sp) in wales and it puts you into the sea instead of on land.
A worse bug is that your units under your command can refuse to do as they are told when assaulting or defending a fortress.
I wanted to defend a choke point and even though I told my men to "hold position" they just refuse and attack the enemy freely and get slaughtered, while some units hang back and continue to defend other locations that I never told them too. If I instead tell them to follow me or allocate a new position for them to hold position.. they seem to make a move to do it, but then seconds later go off and do their own thing again.
Also not sure if this is related, but when defending Chester, in england... I notice that some of my units spawn inside cages, or behind practice dummies. What this basically means is that when you can only command 40 units compared to the enemy commanding 50 units and around 18 of your units are stuck in a cage and behind the dummies, you are only actually commanding 22, against 50. Not sure but I have a funny feeling it was intentional, because I can't believe that someone can accidentally stick some guys in a cage or behind what seems to be a deliberate barricade. Maybe it was put there to give the attacker an advantage ?? anyone?
But anyway, both those bugs make it difficult to defend a fortress... whereas in other mods that don't have those bugs, defending the same fortress with even less men, I win even easier.