I'm moving back to my previous provider: gandi.net (yes, they do domain names mostly, but you actually get a couple of mailboxes with each!)

Why:

• ProtonMail does not follow standards. There is no IMAP/POP/SMTP server with them. So you need to use their homegrown apps, on mobile and desktop to use their service. Granted, there is a webmail, but I don't like webmail, because... offline. So you're stuck with their toys, which are not great. On desktop, they provide the ProtonMail Bridge, which is an app you run on your machine, and interfaces between your email client (Thunderbird for me) and proton's servers.
• The mobile app (again, you can't use something common like k9, as it doesn't speak ProtonSpeak) is a piece of crap. It's slow, there is no offline capability (well, their customer support says that it caches emails once open, which I can't confirm from my own experience). Nope, you read that correctly: it does not do offline. So the email experience on Android looks like: open the app, look at spinner, see list of emails, tap the first one, look at spinner, look at second spinner, read email. Do something with it. Tap second email, look at spinner, look at second spinner. Oh wait, go back to previous email? Look at spinner again. You're in a tunnel, on a high speed train with patchy connectivity? No emails! Or more exactly, emails come in infinitely slowly.
• Recently, there has been some weird boucing happening with emails: I'd delete one email (something you do a lot with Github notifications and the like), it disappears, comes back after a couple of seconds. Then sometimes it disappears for good, or it just stays in my inbox. Both with their mobile app and thunderbird (behind the bridge).
• They do opensource without really doing it. They do post some code on github, but it's not all of their work, and it's more of a "show the code" than anything else. There doesn't seem to be much of a collaboration with users, which I find a bit sad. The issues and PRs seem to be mostly ignored, and most development seems to happen in house...
• Customer support is not great either. As a dev and sysadmin (who runs a mail server), I can figure out a lot of stuff, but it's borderline impossible to convey to them the details of my findings.
• It's not cheap. I paid them north of $100/year for the past two years, for their business offer (which made sense, I was trying to start a business, wanted email that just worked). For that price, which is fairly high compared to what you can find elsewhere, I'd have expected tiptop customer service, prompt response and all.

I can't comment on their security. I follow usual digital security hygiene, but I'm also not someone who's at risk of getting arrested by the police where I live. If that's your profile, maybe the drawbacks I listed above are worth the extra security, but not for me.