I do run PBS in a VM with storage over NFS - and it does work for me. The main problem is to supply that VM with a Datastore which is fast enough; the recommendation calls for direct attached SSDs, not for virtual disks or network filesystems.
So my recommendation is: give it a try. But be aware of possible problems like slow backups and especially slow verify. These problems will increase with the amount of data stored in PBS. It might work first and fail on the long run! Once I was at a point where my installed rotating rust was simply too slow to list the backups in the Web Gui...
If you do run it as a vm, I would also recommend that it be in a different cluster than what it backs up, or at least the majority of the vms it backs up. If you have a cluster wide issue, you don't want your backup server down.