AMD cares to contribute to Mesa. Nvidia? Not really.
It is strictly worse. NVidia not only has no interest in having a Mesa driver, but would prefer if they didn't have to compete with Mesa at all.
Driver support going away after a few years, random issues, etc
Every single nVidia card I used under linux just worked TM (ofc I'm not "pure" because I really couldn't care less about open vs closed drivers)