ProtonMail does not yet have IMAP support (it's in beta after more than two years of requests). So if you ever get tired of ProtonMail and want to move to some other provider, your only option is to save each mail or print it out - one by one. There is no bulk mail export option. Unless one is super confident of staying happy with ProtonMail for at least a year or two (by when I expect IMAP to be publicly available on the platform), I'd stay away from it for any serious use.

Leaving this aside, other factors do favor ProtonMail. The people who run it seem to have their minds and hearts in the right place as far as privacy is concerned.

I'd really love to support ProtonMail but one thing I can't swallow is that there's no server-side search of email bodies (only to/from & subject) because it's encrypted. This kills the possibility of searching through years of newsletters & stuff :/

No particular reason, I just knew fastmail from many years ago and liked them.