> The second limitation is that it is impractical to audit the hardware to determine whether you have actually received the unmodified open source design. For software this can be achieved using reproducible builds or just by compiling it for yourself.

Mostly too expensive AFAIK. I'm fuzzy on the details, but it should be possible to create high-resolution (e.g. X-ray) scans of the chips (as is done by chip design pirates) and compare them to known-good implementations, or images generated based off the chip's open-source design.

I'm looking forward to a future where PC auditing shops are a thing. Take in your machine, and let them verify the contents of every chip and storage unit on your device.