Seems like a good alternative to Dropbox, Google Drive, One Drive, etc, but is it too good to be true?

I've read some good reviews and recommendations about Sync.com and I still have to try it out, but upon creating a free account it really does look like a good service. You only need to provide an e-mail address to start and you get 5GB free storage, while they promise zero-knowledge and strong encryption for privacy. On top of that you can use their app on different platforms, and that's where I get some doubts.

> Sync’s zero-knowledge encryption technology is based on open source software. You can view our browser-based source code at any time.

They make no mention or reference to the source code for any other platforms, desktop, mobile or server side, so I'm not quite sure how reliable this service really is. Can anyone help me with that?