they make a lot of promises about security, but email can truthfully only reach a certain level of security. the comment from @RTRedreovic shows weaknesses in relying in protonmail to protect various aspects of your communications, but they sell themselves as TOTALLY SECURE.
the lady doth protest too much.
so they’re no more secure than, say, google, when you implement your own e2ee on top of email with PGP or something. but the promises of enhanced security actually set people up to expect more than that. coupled with the fact that they don’t even let you use imap or pop, it’s not exactly a hacker’s dream service.