I am a big fan of the dedicated NAS boxes. I am currently using the Netgear RN10200 (which unfortunately is no longer available). If you can find used RN10200's on ebay, they are great as they use BTRFS for the disk and they are basically a Debian computer system. They tend to be very reliable. I have a second NAS running as a virtual machine on a PC I run at home, but the Netgear NAS boxes run at a fraction of the power. The RN10200 use about 17 watts.
Don't touch the older versions of Netgear NAS's that do not run Netgear ReadyNAS 6.x. You want to look for numbers like RN102 (for a 2 disk mirrored NAS) or RN10200, RN104(4 drive), RN212, RN312, RN412, RN414, etc. Numbers like that. The ads sometimes have the extra 00 on the end and sometimes not.
There are lots of other good brands. Synergy are superb, but last time I looked, the budget models were a different OS and I would not use them. I think the good Synergy models can run BTRFS.
The other big brands I am sure are also excellent.
You want a NAS that has a file system with a genuine snapshot ability, and redundancy in the drives. BTRFS allows you to scrub the drives regularly that basically checks every bit of data on all the physical drives is correct. I have had BTRFS NAS's save a client from cryptoviruses a few times now. When I set up NAS boxes for companies as a backup, I turn off windows networking and use rsync for all the synching from workstations and servers. Rsync does run well on Windows and it is completely free - you just have to know how to pinch it along with the few DLL's it needs from a cygwin installation to get the latest rsync in a portable form.
Raid has been the main weakness of BTRFS, and Netgear uses the venerable Linux mdadm RAID system instead and runs BTRFS on top effectively as a single disk drive. This arrangement seems to be rock solid.
One NAS is not a backup. Ideally you want the NAS contents synched to an offline location - like the cloud, relative's house, etc. I use rsync to provide the synching. Quite often, I am synching terabytes of data over a humble ADSL line.
Richard.