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 :/

[I'm stupid they have 2FA, ignore this comment.]

Another huge problem is the lack of two-factor authentication. How can a security-oriented service not offer TOTP or even SMS 2FA.

2FA has been out for almost 6 months.

Oops, my Google-fu is worse than I thought.

You will be able to use the upcoming ProtonMail IMAP/SMTP bridge to do fulltext search locally.

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

I emailed them (Proton) on few occasions with important questions and never got a single response. That alone turned me off. Will go with Fastmail very soon moving my yahoo since Verizon bought them.

Where did you email them? They're known to have one of the best support out there.