Unfortunately, it is only partially cross platform (no Linux client) :-(
Sync to a compatible VM or server and then set up rsync?
That could be doable, but requires setting up an always available server in my home and installing Windows on that - not exactly the best of ergonomics
Does it support differential sync? It's the only reason why I'm still using Dropbox.
The Sync apps provide end-to-end encryption, so the entire file is uploaded when it changes. Agreed differential sync can be a good thing, especially with super large files that change often.