I'm using WebDAV to sync files from my phone to my NAS. There weren't any good alternatives, really. SMB is a non-starter on the public Internet (SMB-over-QUIC might change that eventually), SFTP is even crustier, rsync requires SSH to work.
Syncthing is great but it does file sync, not file sharing, so not ideal when you say want to share a big media library with your laptop but not necessarily load everything on it
What else?