I’ve been testing Syncthing as a replacement for Dropbox and so far it’s been great.
I am currently syncing over 25,000 files in 5 directories across two Macs and one Linux machine. It has worked nearly without a hitch. I say “nearly” because after changing the case of a few filenames those now show as “out of sync”. They synced fine but show as unsynced. I assume this is due to case-sensitive vs case-insensitive file systems. I’ll need to figure this out but it’s more an inconvenience than a problem1.
I’m still using Dropbox for things I’m sharing with others, and probably always will, but for things I don’t want anywhere near a “cloud”, Syncthing seems like a perfectly fine solution.