Also worth noting that I don't think I experienced hard fails, it's often the unrecoverable error count shooting up in more than one event, which tells me it's time to replace. So I don't wait for the array to be degraded.
But I guess that's an important point, monitor your drives. Synology will do that for you, but you should monitor all your other drives. I have a script that uploads all the smart data off all my drives across all my machines to a central location, to keep an eye on SSD wear levels, SSD bytes written (sometimes you have surprises), free disk space and smart errors.
