I have it installed directly on one Proxmox-VE host and installed as an unprivileged container (with a host mountpoint) on another. Both work fine! Few things I noticed:
1. If you have either a Proxmox-VE or a PBS subscription but not the other, you might have strange updates because one uses the enterprise repository while the other uses the no-subscription repository, which will be out of sync about which kernel version to use etc. (this is why I used a container).
2. If PBS is running in a container (on ZFS), it can do snapshot backups of itself without problems. This surprised me in a positive way. I can just backup all VMs and containers, without having to make a special exception for the PBS-container. (I sync the backups to the other PBS, in case the host or container breaks.)
3. Making snapshot backups of running VMs is faster than non-running ones, because QEMU keeps track of changed disk blocks while it keeps running.