Excellent point! I do run my main PiHole in a VM, but I have a second PiHole running on an RPi just in case I'm rebooting the hypervisor. DNS is the only real dependency that can cause problems for me when the hypervisor reboots for updates. I'm just running Fedora on a somewhat recent Dell rack server.

Firewall is a great example of what not to run on a vm, at least for me! I consider gateways as appliances though, and I haven't run my own router on Linux since I first got Cable internet in 2002. I remember how awesome it was compared to the weak routers available at the time!

Now I just buy a UDM Pro and forget about it!