I have been using two Synology CS406 for a whopping 6 years now, no problems at all.
One of the Seagate hard drives died last year, they were a small fortune at the time I bought it but now you can get them for a few peanuts. And my photos are still intact (RAID5). I made a backup in read only mode before replacing the drive, just in case. But i didn't need it at the end. New HDD in, Synology started rebuilding, everything good again.
The only complaint I have with NAS is that they are so damn slow. Even gigabit ethernet can be slow if you are transferring backups with dozens of gigabytes or HD videos (a simply iPhone HD video of a few minutes has a gigabyte these days). I know some of the newer models support port trunking (so you can combine two gigabit ethernet interfaces), but I think you'd also need the right router/switch for that. And it would only speed things up if you also have two NICs in your computer which you also trunk (so no go for notebook). There should be a NAS that has a USB 3 port or SATA port as sell, so that you can access the drive via that. But it seems not to exist. To be fair, a server would have the same limitations. The only thing I can think of would be a DAS attached to your main machine, which also serves as files server.