At that point just run the code inside a chroot with a full /dev and call it good enough. No common GPU driver, firmware or hardware was designed to securely run really untrusted code from multiple tenants.
The "Linux hosts Linux" case does seem the least interesting for that reason. I hope one day this results in actually usable acceleration of hosting a windows VM.
WebGL / WebGPU are a somewhat safe subset. Or at least safe enough that Google will keep funding multi-million pwn2own bounties for Chrome with WebGL / WebGPU enabled.