For me it was search. The proton apps are the only way to access email on mobile, and on them and on their webapp the search barely functioned, even with full text search downloaded. The only way to reliably search my email was with Proton Mail Bridge on desktop, but for some reason it continuously was using CPU on my laptop and of course didn’t work on mobile. If they made a server version that I could put in a docker container on my server it would probably solve most of my problems with Proton, because then I could access it from the Mail app on my phone over IMAP.

As a Proton user - the main trade-off for me is that you are forced to use their apps on mobile, and those apps are pretty barebones and (on iOS at least) have none of the bells and whistles of a modern iOS app, such as Home Screen widgets.

Since I use my own domain for email, I am considering moving over to another provider once my subscription term is up. I really miss widgets.

for me there were two:

no caldav support so I couldn't get my next appointment as a widget on my phone. Similarly, your contacts in proton are trapped there and cannot sync with any other system (such as your phone...)

limited quantity of aliases compared to fastmail. this is actually a really sticky feature with fm from what I've been seeing. I would have to rename a bunch of accounts or switch to using a catchall to transfer out.

When I cancelled Proton the subscription ended immediately instead of running out the clock on the time I had paid for. Left a really bad taste in my mouth.