It does not support [0] (i.e. loses) extended attributes even between systems that share support for them, which in the case of macOS means no support for Finder tags, custom icons, legacy Mac fonts and countless other Mac features, maybe niche to most, but still important to those that make use of them. AFAIK it is not even in the roadmap.
[0] https://docs.syncthing.net/users/faq.html#what-things-are-sy...