I think the biggest issue is, at least in my game, kingdoms are always at war with each other. I joined a kingdom as soon as I hit clan level 2 and that kingdom was at war with another kingdom even before I joined. Many in game years have passed yet they still haven't made peace. You can't even suggest them to make peace using your influence. That option is grayed out. My guess is, they will fight until one of them simply disappears.
Even in Warband, kingdoms would make peace after a period of fighting. This is why snowballing would take a lot of time. It wasn't like in Bannerlord, where I am sure same two kingdoms will keep fighting until one of them is completely gone and the winner will immediately start fighting another kingdom.
Correct me if I am wrong, but in Warband, losing kingdom would send peace offers and winning kingdom would sometimes accept, I doubt this is happening in Bannerlord, because they always fight until they die out.
This is a weak point of Bannerlord. AI doesn't know how to surrender. You can have a 500 man army and fight against a group of 5 but they are not smart enough to be scared, you can shout "SURRENDER OR DIE" again and again, but they will threaten you and still try to fight you. It is ridiculous.