When you try to enhance the psx graphics by using 3d hardware certain glitches appear. So it's a trade-off: you get better graphics, but sometimes you also get messed up graphics.
For example: R-type delta. If use pete's Opengl I get messed up graphics whenever I pause the game, but everything is perfect with the soft plugin
The only way to get a 100% compatible GPU plugin is to use a software rendering engine. Unless you build and market a graphics card that has a 100% hardware-compatible PSX GPU in silicon
Any hardware GPU plugin has to make compromises to the graphics, at some time. To understand this fully, you need to have a good knowledge of programming PC 3d accelerator, and a good knowledge of the PSX GPU. Then you will understand the compatibility issues between the two.
If you don't have this knowledge, then trust me