I wont go in to what platform, how much performance you loose running what on where... but I must say running OMV (or anything else for that matter) as a VM makes backup and configuration much simpler.
If you need to change something that you are not so sure of use a snapshot or checkpoint or what ever you wanna call it, make the change and if things go bad just go back to the snapshot/checkpoint..
If the VM is backed up everything in that VM is backed up and configured just like it was when the VM was backed up, so a restore operation of the whole thing (or moving to different hardware) is sooo simple.
But there are also downsides to this, usually when you backup a VM its hard to exclude certain things from the VM that you normally would not backup - so if you have a large scratch space, temporary files and what not - those would be included too...
I run OMV on my Windows machine as a Hyper-V VM, every week I have powershell script that backs it up to another disk and that works just fine for me.
Then again, I don't have any specific needs for performance and do not need to get everything I can out of the hardware I have, just a simple OMV that shares some files and some light containers that run pretty simple things.