Basically, they still ultimately rely on trust, just like others. While they're drastically different from e.g. Gmail, trust requirement is still there.
If one's well-being really depends on security, it's best to err on side of caution. Meaning, something like MailPile on the client side, very cautious non-automated key exchange, and, optionally, self-hosted email with at-rest encryption (e.g. Postfix+Dovecot+GnuPG pipe) is probably much better option than anything Protonmail can offer in their current state. Or avoiding email (as it's not yet there and probably never will be) and using something like Wire or BitMessage or whatever (I'm not a security expert) has both client-side static and auditable source code and limits metadata leakage.