I've been with them for over a year now as well and not a single issue. The interface is a bit cumbersome in some ways, it's a customized OpenXchange, so some mailbox.org-specific settings are done outside of the rest of the more general OpenXchange settings.

What you get with mailbox.org that you don't get with ProtonMail:

  * IMAP and SMTP
  * Support for hardware 2FA dongles like Yubikey
  * Ability to use your own GPG keys
  * Very capable mail filter config tool
  * Calendaring and contacts (with nice URLs so it's easy to use on Android via DAVdroid instead of Google's stuff)
  * File storage
  * Web-based spreadsheet, word processor and presentation tool

Their jurisdiction is Germany so you get whatever is left of EU privacy laws plus the Germany-specific ones.