There are corner cases. For example, icon files traditionally hold all the icon data in the resource fork on Macs, because the icon is a “resource”. Syncthing will happily sync this as a zero byte file to everywhere else, including other Macs.
My personal opinion is that today it would be an extremely unwise choice to store critical data as alternate data streams, in resource forks, etc.
Nonetheless these things exist and it would be neat to preserve them. Pull requests welcome, as noted multiple times above.