Earlier quoted context omitted.
Pi-top like laptop with your choice of pi3 or BeagleBone running Linux. The performance of a pi3 is actually decent. It's not perfect as there's a GPU BLOB in the pi3 and the BB also has some issue. It's my compromise for now, hoping the blob will be reversed/replaced eventually. Or anything that runs libreboot: https://libreboot.org/docs/hcl/ If OpenBSD runs on it that's also a good sign usually as they won't touch…
I do actually on a Pi3, so that's an encouraging piece of info. A GPU blob is at least a step up from the ring-negative-3 management engine.
The GPU firmware runs in parallel to the CPU and has access to the complete memory. Video decoding is done by the GPU and happens while the CPU is completely idle. And it can (of course) crash. If you've done anything related to OMX programming on the Pi, you might have experienced that.
In theory there is nothing that would prevent the a rogue firmware from hooking into kernel structures to interface with the outside world.