It's mostly issues you'll run into once you're into the mid-game. Economics issues and realm management issues mostly. Since social events aren't really a thing in the game (like feasts, tourneys, and weddings), you don't really get a chance to raise other rulers' disposition towards you. Warband had a stat called right to rule, which you could raise actively while being a vassal, preparing you for when the time came to declare independence. That helped you gather allies quickly, depending on the deeds you had done during the playthrough.

In Bannerlord the only real way to raise disposition quickly is to be in the opposing side of the lord you want to raise your disposition with, by beating them and letting them go. Voting in their favour in kingdom decisions yields pretty meager returns.

However, the biggest issue is economics. Mid-game/late-game there's very little ways to ensure you can keep a big army fed. You're almost forced to level smithing in order to smelt down anything valuable the enemy had, forge new stuff with a higher quality, and go around emptying cities' pockets through the same 4-5 minmaxxed weapons.

I really want to love the game but its really only fun during the first, what, 10 hours of a playthrough. It turns into a real slog because there just isn't any diplomacy.