You can't use any 3rd party mail app on Android and the official one is really bad (no thread view, fails to send emails to some valid addresses…). Also, you can't two-way sync your contacts. And the encryption thing is not really useful since too few people are on Protonmail (so all your messages will be stored in plain text somewhere) and Protonmail will share your data anyway (see https://news.ycombinator.com/item?id=28427259).