Interestingly, I've always found with PSX emulation that 2D seems more of an awkward thing for an emu to get right than 3D... and then we have the fact that a 2D picture is actually still using a full 3D engine to display a polygon, just so close to the camera as to appear entirely 2D.
Does make me wonder if some of the really simple 3D games/demos might actually be easier to pull off than 2D games.