I'm not so much saying it's a bad idea as that what my parent comment described was a logical contradiction. It isn't possible to run "anything that is susceptible to outside data" in sandboxes, because that makes the sandbox susceptible to outside data. If you're genuinely assuming that anything susceptible is already compromised, then the sandbox is accomplishing literally nothing.
Yes, microcode vulnerabilities is a problem indeed. Hopefully Qubes Air (next version 5.0) will compartmentalize even that by using separate devices as qubes: https://www.qubes-os.org/news/2018/01/22/qubes-air/.