I'm successfully using rpi-clone on almost all my Raspi, where I regularly swap boot/clone SD to check the last backup will work in case of boot SD I/O problems. I'm very satisfied with rpi-clone's system backups.
On OMV, booting the rpi-cloned SD gives problems with the Raspberry PI 2/3/4 OMV distributed image (Armbian), partly because some /var/ subforders are lost maybe in the "rsync --with-root-excludes" process.
After booting with the cloned SD, JellyFin works regularly, but no OMV web UI. I noticed an error in one openmediavault-*.service, so I manually created the missing /var/log/openmediavault/ and executed omv-initsystem, but still no luck.