Very honestly and frankly, I have not given much thought about it. First NAS I purchased was a 212j and I took the risk as there was a huge price difference, plus its a hardware board, embedded system, it rarely goes bad like this, unless you are that unlucky.
Reviews were also positive and I could not find any post that stated the piece entirely went bad after use. Touch wood all went well, the second NAS was DS413j that Synology themselves sent for review, that too was via AMAZON route and I have never faced any issue with it, mean time other people on hifi were also purchasing the same way so the confidence grew, the latest one 916+ I have also was via Amazon (even though I purchased it from a FM) so as I said frankly never gave a thought much bout the other side of the story. But if things do go south, I would probably go to an authorized repair center and pay to repair, that is the only downside I can think off, but with confidence on this device and huge price difference, it's worth a risk .. IMHO..