I heard the Saturn was originally designed as a 2d machine, sega saw what the PS1 was going to be capable of and bolted on 3d. Hence it was a mess. Is it true it could only use quads not triangles for polygons?
Yep, but obviously a quad could become a triangle with two shared vertices. I never worked on the machine myself, and my memory is definitely fuzzy on this, but I seem to remember one of the things stressing out my colleagues was the poor transfer rate of getting textures into the video hardware, and the space being quite limited. So, all textures ended up being down-res'd by 50%, which just made everything look terrible.
I'm sure there were other issues, that's just the one I can vaguely remember.