Signal is a good example here because someone did a usability study. In a usability study involving Signal[1], 21 out of 28 computer science students failed to establish and maintain a secure end to end encrypted connection. The usability of end to end encrypted messaging is a serious issue. We should not kid ourselves into thinking it is a solved issue. For all practical purposes it is the issue.
[1] https://www.ndss-symposium.org/wp-content/uploads/2018/03/09...