Joanna's (Qubes OS Founder) blog  is a gold mine when it comes to hardware-software boundary security. Especially "State considered harmful"  and "x86 considered harmful"  papers are eye-openers.
Please read x86 considered harmful by the security researcher Joanna rutkowska. It highlights the x86 flaws. https://blog.invisiblethings.org/papers/2015/x86_harmful.pdf
I think the whole point of Qubes OS is t not trust hardware because of potential BIOS or ME backdoors.
Joanna Rutkowska, Qubes founder, is the person who brought up intel ME as a problem in her paper Intel x86 considered harmful (https://blog.invisiblethings.org/papers/2015/x86_harmful.pdf).
The article is dated.
The game changed fundamentally with the introduction of the Intel Management Engine (or the AMD Platform Security Processor) on the x86 platform. The system is now "deep pwned" as described in point 3.1.1 of the article. The manufacturer has ultimate control of the platform, the user has been disowned.