I didn't start playing MnB until about 1 year after Warband was released. It was an enormous learning curve, but I could tell it was worth it. If it hadn't had that depth, I'd have stopped playing after a few weeks.
Warband is far far better experience for casual players. You always understand what's happened and can analyse the matter and learn. You can always skip a better player by running away, gang up on him or throw stuff at him to wear him out. You always have a chance to get him/evade him in a huge chaotic melee. Also, Warband have turncaps and blocking feels faster and more reliable. And madskillz are kept away on special servers where they wreck each other instead and not pugstomping. I still enjoy every second of it and go there from time to time to heal my nerves after Bannerlord.