I have been a ProtonMail user for a long time but have yet to make it my default email provider. The main problem is that I keep an email archive of all my emails: all sent and most received since 1995. It is 17Gb. I've imported it into Gmail, and it is instantly searchable. With ProtonMail, such a search must happen on the client-side due to encryption. I suspect it will not work very well.