I prefer vanilla Warband (the only mod I ever used extensively was Diplomacy) over Bannerlord because the core of the game, for me, has always been combat mechanics. Warband is simpler, but because it is simpler it is more feasible to do complex things. The evolution of technical ability with Warband's melee mechanics in the Native MP dueling scene was fascinating to me, as a participant and observer. Skills translated 1:1 to SP, though bots would obviously commit on every error.
The key difference for me is the absolute nature of directional blocking & attacks in Warband, as opposed to the variability in Bannerlord. In Warband, blocking up will block all vertical attacks from a wide angle in front of you, regardless of height difference between you and your attacker. The same goes for blocking down. Blocking left and right offers the only exception, where a "wrong" block on one side can still intercept an incoming attack if your "wrong" block is rotated enough to face your attacker. This seemingly odd interaction makes perfect sense when seen in action, though, so it feels natural. By contrast, BL requires correct block direction and intersection of weapon/shield hitboxes to score a block, which means correct timing, input, and facing can still result in a failed block due to terrain or character height difference or short weapon hitboxes. Mixed results from correct input feels bad in skill-based games. The same distinction is made for chamber-blocking, which was so straightforward in Warband that it simply became a way to disrupt timing, and is so finnicky in Bannerlord that almost nobody makes deliberate attempts to do it.
I played M&B starting with the .623 beta for the original game in 2005, was in the first wave of invites for the Warband closed beta in August 2009, and participated in the friends & family beta as well as the closed MP beta for Bannerlord. I've played thousands of hours in the first game, and in Warband both SP and MP where I was deeply involved, but I can't stomach Bannerlord for very long. It does not feel as good as its predecessors, even though it looks better and has some interesting new systems.