3 reasons as far as I can tell (in no particular order):
1) old/weak PC - used to run Warband in DX7 mode
2) misconfiguration - "woah new buttons and sliders... better turn them on and all the way up. wtf my game runs worse than before for no reason wtf is this shit noob devs"
3) bugs - mostly rubberbanding/stuttering issues. Some are regressions, some others are caused by new options (not always easy to tell - see point 2).
1) Okay, I hadn't considered dx7 vs dx9. So that's a legitimate compatibility issue with wse2 itself then? I wonder if the performance improvements it offers are enough to compensate for older weaker machines to be able to run in dx9 and have it work, even if on lower graphics settings. Or is it just impossible for certain systems to set to dx9, the option isn't available at all or something.
2) That's still my guess for probably most of the issues, though I was thinking it was more not enough tweaking, not over tweaking as you suggest, but maybe that's possible as well if they do it in the wrong direction. A comprehensive guide would be nice. I'd been thinking about sharing my settings but since every computer is so different and are running on such different specs, it's probably not much use.
3) All right, so along with 1) this could actually be problems within wse2 itself, but which only presents themselves on certain specs or configurations. Although rubberbanding/stuttering sound like they could happen with or without it, isn't that a connection issue? There's always a chance of that it seems.
My biggest tip would be setting to 60fps, that's what made the biggest difference for me as I recall.