I had no problems with the Win3 UI and was never into using themes, choosing background/window trim colour was fine. The Win2k one looked newer. The only real benefit was that there were some new admin views. I mean look at modern file/web browsers, they don't have much in the way of raised/sunken UI elements.

I was a developer so cared more about performance and stability. How my code editor windows looked didn't matter. Memory being taken up by a fatter UI implementation that slows things down counts against it.

Edit: and yes, win2k/NT4 was less stable. They moved the graphics subsystem into kernel space where as NT up to 3.5.1 had graphics as a separate service. And you can imagine that a lot of the bugs/edge cases in Windows is in the GUI and drivers.

On NT 3.51 you can install the NewShell update and the UI becomes more like Win95