Sure it is not the best tool -- but depending on what means "critical" in your context it provides great security. Not against the NSA targeted attacks, sure, but if you're not so concerned about your adversary using exploits and more sophisticated tools (versus just demanding data from the provider or something like that) ProtonMail goes a long way.
Anyway it is, for most cases I can think of, much better than Gmail.
Anyway it is, for most cases I can think of, much better than Gmail.