It's great to hear mappers enjoying this! To me it's like moving art or magic to see such large-scale projects rendered in a primitive un-smoothed way. gl_nearest comes close, but it's not quite the same...
Clipping issues result from limitations of the renderer rather than vis. One type occurs with map situations that would be discontinuous in the real world, such as entities or geometry behind sky. The vis tool can't hide all of these surfaces, but with hardware rendering, or standard scrolling Quake sky in software, the sky masks therm. Another problem looks like entities Z-fighting with walls when the distance is great. Z-buffer precision? I'm not sure. A third type that has been fixed AFAIK looked like gray rectangles that flickered briefly. I'm pretending that it's impossible or too computationally expensive to fix the other issues
