None whatsoever. Arguably these have better QC since they've been checked over.

The only potential issue you might run into is if you have a crazy amount of read/write like you're running a corporate server or something. (Drives have a set max on number of read/writes and refurbs dont always reset this counter). This is absolutely not an issue for the average user who is doing backups though. You won't get anywhere near the limit. I wouldn't run a typical server off a NAS anyways since speeds are too slow.