In terms of backups, Roon has it's own automated backup solution built-in - you set up your schedule and point it at your backup locations, either locally or on your network (so e.g. your NAS).
Those backups are only the database, not the music library. You will still need to maintain a backup of the music library.
If using external USB, that can be a simple offline copy of the music folder from the drive, or, can be done whilst online via a network copy using the NUC as a network device.
If using internal drive for music, ROCK takes over the drive management. So even forgetting the not so straight forward access to the drive, access to the music folders is not quite as simple as just being a generic music folder that you can pretty much do what you want with. If you stick with the external local USB option vs remote NAS, you also get benefits from the music library being instantly to hand, such as when undertaking content changes to the library.