With a standard Linux system you have vulnerabilities X,Y,Z. With Qubes you have vulnerability X, so let's comment on Qubes and try to discourage people from using it because vulnerability X still exists? It's still better than the alternative!
If the ring architecture of processor can be circumvented, the protection of privileged code (kernel) is harmed. If you can circumvent the MMU's memory protection, the protection of privileged code is harmed. And if side channel attacks like Spectre and Meltdown are circumventing protecting features, the protection of sensitive data is harmed.
Every systems which has claims on security relies on explicit (that would be better) or implicit assumptions. And every operating systems I know of, at least implicitly assumes that the hardware isn't compromised.