Lords are too independent and oblivious in Bannerlord. They make almost no effort to cooperate defensively so basically as soon as one faction can somehow muster an army above a critical point (usually 200+) they can run into enemy territory with only small parties of 40 or so to meet them. No army is mustered in response and every lord they beat is dumped back into the world alone to be captured several times by looters and mountain bandits before they can slowly recover their modest sized party, if they ever do before they defect.
I like the idea of smaller lord parties and lords having to cooperate on campaigns, but there needs to be better defensive cooperation. Each faction should have certain "heavyweight" lords who take leadership and muster armies in response to enemy sightings/attacks. As it stands it seems armies are only mustered for offensive campaigns.
As well, the army system is an improvement (in theory) to the marshall system in Warband but lords don't seem to be able to cooperate outside of it. I've watched 3 40-man parties of Vlandians take turns running away from a party of 60 Battanians raiding a village. The only time individual parties reinforce one another is when they're coincidentally near each other when the battle is started. There needs to be some way for nearby lords to recognize a threat and automatically form an army if they can muster enough men to deal with it.