There's the impression that Protonmail isn't quite as private as it advertises itself to be. Some even go so far as to call it a compromised service. I agree that there are some red flags, such as how one of their dissident users got arrested earlier this year because Protonmail gave law enforcement their IP address. This especially alarmed me because they make it difficult to access the service via Tor - or at least, so I thought. It's true that the clearnet URL won't allow you to register behind Tor unless you provide a phone number, and you have to solve a captcha each time you log in. However, there's also a hidden service where both those drawbacks are absent. So you can, in fact, maintain your anonymity with Protonmail. I find it noteworthy there's such a lack of awareness of this URL.