In a desktop, couldn't you assign a GPU to one video machine and in that scenario would there still be a security problem when there is only one VM using it?
It’s not about virtual machines. GPUs typically have direct memory access to pretty much all system RAM. There exist PCIe mitigations, but the review does not meet up to Qubes security standards.
The drivers are fine for GPU accelerated rendering of the app surfaces, even on the Pi. Hell, the drivers are even there >98% of the time for accelerated decode of the video format itself to boot.
Qube's unique choice in software only rendering for user applications is one born out of the isolation goals for security, not what the software/drivers/hardware could do.
No, as there isn't a non-free driver alternative to compare V3D to. For VC4 (Pi 1-3) there was a proprietary alternative driver... but it was a bit garbage.
It feels you may be conflating the "proprietary firmware blob on the GPU which is used to boot the Pi" story with the GPU driver itself.