Protonmail would be great with some usability and feature improvements. Right now it has limited support for imap, smtp, pop3. You're basically limited to the biggest desktop clients. The desktop web client is okay.

But the biggest drawback right now that I can see, is mobile. Their app does not support threaded conversations or any of the newer features that we've come to expect because of Gmail. You also can't use a third party app like k9. So for me at least, it's not good enough yet to use as my primary account. Really wish it was though.