All above, plus: System requirements — Qubes OS 4.3.1 documentation and How to install and use Qubes OS inside VirtualBox
I’d be interested to see how useful virtualizing Qubes for automatically testing certain aspects of Qubes without bare-metal. And I mean only certain aspects like “do the salt scripts not fail in a fresh install?” or “do the upgrade from 4.3 to 4.4 not fail”? Like a first-pass type of test that can be executed in a few minutes and created from scratch. Won’t replace bare-metal testing.
I would argue that not controlling the hardware being the largest issue to privacy. As far as the x86 VM is concerned, it control’s ring 0 (a virtual one) and privileges are respected inside the VM; off course, it’s all virtualized by the hypervisor and correct as long the hypervisor correctly implement such controls, but is just an illusion of hardware control. It would be my guess.