VFIO doesn't work with Nvidia devices because of a design decision by Nvidia. Their drivers refuse to load in a virtualized setting unless you buy their enterprise GPUs. You can patch out a few checks and everything works again. Another option is to reload the GPU firmware before the virtualization host boots so the GPU doesn't know it's the second time it boots.

Even if it's possible to implement proper VFIO for Nvidia GPUs, I expect them to fight such an implementation with tooth and nail in their drivers. This is the same company that artificially limits the amount of transcoding streams in their drivers, mind.

VFIO is already possible with amd if you have supporting hardware i.e. i was running single vega guest windows guest and linux furyx host on tr4 board but with sr-iov you could run many guests using same gpu, i think stadia uses this now. There were also rumor that vega has sr iov but disabled on hardware