While you can do a lot of the stuff that PM does via Unraid and other tools, it’s all there in one spot. I love taking snapshots before upgrades, migrating machines between nodes live while I upgrade the nodes, having HA for my OPNsense and other important boxes, and the PBS backup system. I know you could do all this with other tools, but it’s damn convenient in PM and “just works”.

You can install a NAS vm in PM, just give it raw access to the disks you’re looking to use for data, and back them up independently. Don’t try to do something like overlay ZFS on ZFS.