It seemed nice at first, but one major issue: GPU passthrough was a nightmare. It cant be done in the UI and I didnt understand fully how it worked. There are many different tutorials not by promox that are outdated or may not work. It was frustrating enough I jumped to NixOS. Other hiccups included having to go to the terminal to passthrough drives for openmediavault, but that one was kind of straightforward atleast, and it worked first time.
In hindsight, I didnt actually need to virtualize everything at that level, so I never really had a good use case for it anyway. I use containers over entire VMs.