Wow, now I'm even more confused. This is my understanding of the situation:
Data is very easy to understand/deal with - and presumably it wouldn't really matter much which file system its in as you can just read it back with the native system (so if the file system is ext4, use the NAS or linux, if it's NTFS, use Win, etc.). The data is discrete - obviously it would be preferable probable to put it in the filesystem which you're more likely to read it through...
When it comes to OS restoration though, its gets trickier
To zoom out - I have multiple backup solutions in use in simul - I have an onboard "bootable clone" of my OS drive, so if the primary fails, I shut down, reboot using the clone, seamless restoration (with some minor delta loss from whatever wasn't backedup to the clone in the interval before failure of course). This is essentially the 'external drive' that's been referenced, only I have it on SATA so it can boot easily.
I then have an NAS (aforementioned), which serves several purposes (yes it shares network files, etc.) but which is also a backup unit (I know that this is certainly a common practice). What I seem to be hearing is it's a backup unit EXCLUSIVELY for discrete files (and NOT operating system backups).
This makes some sense to me (while at the same time I'm becoming more and more convinced that all of our computing architecture, an OS with an MBR, discrete files, different file systems, is primitive and in need of a massive technological update).
On the other hand, in principle, the only problem with using an NAS in the nature I'm looking to do so is in the file system. If Windows could read ext4, for instance, I could easily pull the drive from the NAS upon primary OS failure, boot the system with whatever media (Acronis disk e.g.), and point to the image file on that drive (attached to the primary machine).
It could then restore the image, and the backup incrementals over it. I know this could work obviously, save for that Linux file system.
1. I have been reading around, and I've been getting the vibe which I can't confirm, that other NAS models DO support NTFS formatting - if this is indeed the case, then I have simply purchased the wrong model of NAS.
2. Similarly, I have investigated simply changing the OS which runs the NAS, but so far I haven't seen anything on that.