Many HD refurb does are reperforming calibrations, map out bad sectors, etc. Many times things that were sent back weren't really bad, just user wasn't able to run it due to loose connectors or compatibility. Or sometimes they rebuild it by reusing the shell, cover, or any parts that weren't bad.
But think of it this way, not all diagnostic can catch intermittent issue, so there is always a chance that you didn't fix what was broken. With the same warranty length it will be ok, but with only 30 days, it is not really a good deal.