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.
Most of the modern files like office documents and pdfs are compressed. Delta Sync only will help in scenarios if you have bitmaps and vm disks.
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.