Their server and most part of their software are closed source. Only the UI and some libraries such as search modules are OSS. I am not against Closed source software but I am against false marketing. No windows or Linux client means the vast majority of people will be using the web interface and the server could technically decrypt the messages hence breaking their promise of end-to-end encryption.
I would stick with Protonmail or Tutanota and use Gmail/Dropbox with Cryptomator.