Another important question to ask about any NAS:
What happens if the NAS Enclosure itself gets fried?
That happened to me.
My QNAP TS509 died within the first few months of use. I sent it back for replacement and was able to simply stick the drives from the failed model into the new one and they worked as though nothing had changed.
I now keep a spare TS509 box available for such issues.
Can I put the Disks inside another Manufacturer's NAS
OR
Should I buy another NAS Enclosure as a Backup?
That's probably less likely to work as it's likely there are proprietary settings on the drives...