> Not exactly a database but for example with ZFS it is known that ZFS can't provide guarantees that usually provides if you run it in a VM

This is true for any and all file systems. ext4 can't provide those guarantees either because it's up to the hypervisor to do the right thing when the guest requests it to flush caches to disk.

ZFS just happens to have made it very explicit that it requires certain guarantees to provide the data reliability, but those same requirements exist for other file systems.