It's cheap, open source and if you read about their PGP implementation it's the only one I've found that actually encrypts messages as they come in, as opposed to putting them on an encrypted volume that needs to go offline to encrypt the contents. (if any of that is important to you)
Like has been pointed out, just use an external client K9, on the phone and TB on the desktop and ignore the gui. It's all about the backend with this company and for that they seem to know their stuff. The owner has written books on mail setup.
Have a poke about on their website and you'll see what I mean. The whole point of moving away from gmail is the lack of privacy. These are the strongest advocates of privacy around.
https://mailbox.org/en/guiding-principles/