One downside what totally prevents me from even giving ProtonMail a try is vendor lock-up. You'll be stuck with them with no option to migrate away while keeping historical data. No IMAP, no POP3, not even a proprietary backup format, nothing at all.

One could probably automate a browser to log in, iterate mailbox and dump contents, but that certainly won't be fun.

(Also, no way to use your own pre-existing key, only somehow generating a new one somewhere - not sure how exactly it's implemented and whenever I can trust it all...)