From what I have read, btrfs is stable if used in a RAID 1 configuration. However, it seems that RAID 5... is a completely different story altogether. I'm sure it has gotten better, but as of now, I've not read anything about catastrophic data loss on anything but striped arrays.
Given that it appears to be stable, btrfs does have a few advantages, mainly being able to handle bit rot, as well as snapshots (which are definitely not backups, but they are another tool to help prevent data loss.)
If btrfs is a concern, one can run without it. The less expensive ARM based units don't have it as an option, but they do offer RAID (assuming a more than single drive model.)
All and all, NAS appliances have their use. Especially with offloading stuff that would take up resources on an active machine or VM. For example, having the appliance handle basic Git repositories is nice, as well as DNS caching.