Just came here to say I use dietpi-backup to a QNAP NAS over NFS just as OP is asking for and it does work fine. The high level steps I followed were:
- Created a new Share on the QNAP
- Modified the ‘NFS Host access’ in the QNAP GUI to only allow the IP of the host you want to back up using dietpi-backup and grant RW mount access
- Mount the NFS file system using DietPi-DriveManager
- Run DietPi-Backup
Any problems I encountered along the way were all Storage side permissions issues.
Hope that helps.