Incidentally, I did work on a backup program for Windows and Unix-ish OSes some years ago. There are indeed locked files, there are hardlinks, symlinks, junctions and other types of reparse points that will give HFS+ a run for its money, there are alternate streams, DACLs, SACLs, there is a monstrosity called volume shadow copying, there is a lack of support for created times on some file systems, times that are supported are truncated differently, some file names are not supported here, but supported there, etc. Lots and lots of cruft accumulated over past decades.

But it's not a nightmare. It's just a very large pile of mostly trivial stuff.

Also none of this should require a "complete rewrite" if you are to add support for yet another file system. Saying something like this means that either existing program is a bowl of spaghetti code _or_ that someone is being coy and exaggerates implied difficulties. Are you going to argue with that? Because that was the point you were commenting on.