I have had Proxmox in production since 2015, including multiple clusters with hyperconverged clustered storage, at small ISPs and medium-sized businesses.
As a wrapper/management system for KVM and LXC, I have no complaints -- it is simple and effective (and the price is right.)
However, I have also supported the project financially since day 1, as they also do good work around high availability and storage clustering -- I don't know of any other projects making true HA so accessible at the price.
To sum up, proxmox is a well-maintained product for those who have a moderate to advanced amount of linux-fu; it doesn't really get in the way.
I also run it in my
garage heater home lab which may be a *teensy* bit out of hand.
Feel free to ask more questions,