The technical reason is that the microcode isn't designed to be run in parallel -- it does DMA transfers between main memory and texture memory, and such. I don

The technical reason is that the microcode isn't designed to be run in parallel -- it does DMA transfers between main memory and texture memory, and such. I don't understand why running it on the GPU would help anybody. For personal / drama reasons, the main author of libretro (squarepusher / twinaphex) has a bad track record in the emulation community, but this isn't the time or place to talk about it. Just know that not a lot of people in the scene hold a very high opinion of him.