these issues have been found by me over 10 years ago, and i wonder if anyone else have found them.

3dnow software renderer:
speed gain is significant, around 25-30% faster.
sounds seemed distorted
dynamic lights(flying projectiles, explosions etc) would only light up floors and ceilings, but not vertical walls.

3dnow opengl renderer:
speed gain with both nvidia and ati cards are minimal, only around 5% faster than default opengl. i guess that the video card driver is already making heavy use of 3dnow instructions, even though the original quake2 game didn't support it. maybe one can confirm this by comparing default opengl performance on k6 with k6-2 of same clock, but i don't have any k6 atm.
messed lightmaps: the worst bug i've found, it involves the "gl_modulate" parameter which amplifies the brightness of the lights(but not the whole world). the default value is 1 and is often considered too dark, my preference is 2 or 3 and some players may go higher. with the default 1, there is no difference between 3dnow opengl and default opengl. but once you go up to 2 or higher, compare with default opengl and you can see how messed up the lightmap is. i wonder if the same bug applies to 3dnow 3dfxgl renderer, but have never tried as i havn't had any 3dfx cards.