I use an Icy Box 2 bay raid USB3 enclosure to back up my photos, which I only run when I want to back up a batch of photos so no need for NAS. I am currently looking for more capacity and in the course of doing that I looked at a WD BookDuo but rejected that solution for a number of reasons. I use Raid 1 mirrored, so effectively double backed* up to mitigate against drive failure, this is set by physical dip switches on the Icy Box. On the WD you have to use the WD management software to set the raid you want to use and as far as I could see, you also had to use their encryption software, which reading the reviews would seem to have led to a number of people losing their data!
The only reason I can see for going for NAS rather than a USB is if you are backing up from multiple machines on your network. When I do eventually replace my backup system, it will be similar to the existing, nice and simple - two bay, Raid 1 set by dip switches and a USB 3 or 3.1 connection.
* If you include the HD drive in the PC with the photos on it, it is effectively a triple redundancy system
which some of you out there might like.