A failed SMART test is grounds for RMA.
I had to argue (insist) one time when I had a WD Red that had failed a long test. This was in our non freenas nas that we use at work. I scheduled automated long tests because none had ever been performed by the previous admin.
A few smart tests later I get an email that one of the 4 drives failed a long. Call up WD to apply for an RMA, and he says something like, ohh but you can fix the failed smart test by dong a "low level format". Nevermind the fact it's been impossible to low level format a drives since MFM drives back in like 1980. I assume he meant a full surface drive zero, which is certainly not a 'low level format'. And possibly yes, a zero pass may have forced the problematic portion of the drives to start working again, but on a drive that's under warranty that is reporting it's failing a diagnostic test, why even bother trying.
I was insistent that the drive needs replacing as it couldn't be trusted anymore even if the zero pass did work. We'd never continue using it for reliability concerns. He eventually did go ahead and issue an rma, and I got it replaced. (I even printed off and shipped the smartctl -a output, highlighting the problem attributes / failed long tests)
I know Cyberjock doesn't like Seagate, but in my dealings with rma, they've been the easiest to deal with. It's only been WD I've gotten hassled about getting rma's.
That being said, when my seagates fail, I'm replacing them with WD's as I do think they are the better nas drive.