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.