We have a few Synology NAS (some personal, some corporate) across a few companies and up to the management company and we've ended up only using them as Mac time machine backups, or via SSH and rsync for backing up Linux servers and Windows desktops. The Synology backup apps are very slow and unreliable (our audits have shown massive chucks of backed up data missing) so essentially we've done done away with the Synology OS.
We'll replace them all with servers running FreeNAS over the next few months. Far more under our control and with more confidence in what they are backing up.
Given what we've seen, I doubt we'd trust data integrity of a drive plugged into USB on a Synology NAS.