Not sure I would agree. Arguments can be made either way and depends on a specific user's needs. If cache is big enough, it's one less thing to configure, one less drive to buy, and one less port to use.
using it on cache, the vm has to share drive bandwidth with any dockers that are running.
I've noticed slight throughput gains using vm's on unassigned devices. Nothing to write home about, but I think others have seem more. Is it worth the extra setup (as in, adding another drive, and then clicking a new destination for the image file in vm setup)? That is up to each user. But if a problem arrises on the vm physical disk and it needs to be replaced, you don't have to stop the array to do so.
and IF unRaid allows vm's to run without the array being spun up in the future, my suspicion is that it will only be run on mounted unassigned devices.
But you are 100% right, arguments could be made either way. If you're using cache drive with a few drives, then the vm is saved on several disks, and if there is a drive failure, it is still operable. A benefit for some. Maybe we should do a poll, because my initial post was based on what my impressions were on the board. Could be interesting, I could be flat wrong!
On the other hand, I don't actually run any VMs and if I did I don't have room for video cards or additional drives in my server.