When I decided to ditch google a while back I considered switching to proton mail. Their marketing resonated with what I was looking for. After some thought I realized that email is fairly insecure by design. Even if proton mail fixed all of the security issues associated w/ email it all goes out the door the moment I communicate with a non-proton-mail address. Almost all of my friends and family use gmail, and most of the volume of email I receive comes from businesses. For my usecases, proton mail is basically security theater.
What's worse, proton mail makes many dubious claims. They claim that "All emails are secured automatically with end-to-end encryption." This is clearly false. They state that "ProtonMail's infrastructure resides in Europe's most secure datacenter, underneath 1000 meters of solid rock." Ok, cool, but how does that benefit me? The emails are already end-to-end encrypted (but not really). Am I expecting commandos to raid a datacenter and steal my encrypted emails? They say that "Our story begins where the web was born, at CERN." Again, who cares?
End-to-end encrypted email is not on my list of must-haves (or even on my list of wants). When I need a secure communication channel, I use Signal. Proton mail overstates what they provide, and they spend a lot of effort on frankly useless security measures.