It’s been years since I bought one, but there was one issue that I wish I’d have known before buying: Some NAS store the data on the disk using some proprietary filesystem / encryption. Raid protects you from disk failures, but if the NAS itself breaks, you might not be able to get the data off the disks unless you put them in a replacement NAS that is compatible.
That NAS I bought 5+ years ago is long out of production. If it ever fails, I don’t expect I’ll be able to get the data out.
Also, I’ve never used it for anything but windows / linux network filesystem. Those consumer NAS are full with with weird features that don’t really have anything to do with storage. You won’t see a professional NAS with itunes support. But maybe you have use for this. I just wonder how secure my NAS is with all those unused features that never got security updates.