I used proton for 3 years and even had the former visionary plan.
As already suggested: no IMAP SMTP so you can go screw yourself with a mail tool unless you install the bridge app, which will only accept local traffic so no fiddling it to your phone. (Only protonmail app)
Then you pay strictly per user/alias which was a problem for me and my company, because i had employees listed who only look once a month but still block a slot.
Also no possibility to add system accounts due the lack of imap smtp, so no webshop daemon, no newsletter tool, no hubspot integration and so on.
Their hole privacy thing is a nice facade, they state they encrypt 100% of the mails, however they are really happy giving out information on what they have, to whatever authority is knocking. Reverse google their name and look for the news…
My biggest and baddest problem was their engine. The search was absolutely crippled, it sometimes took ages or said no results, only to be re-run to finally find it. But it only searches the subject, not in the text.
Then there were repeated outtages in September or october 2021 (i think).
Either no service at all or slow as hell (dont ask about the search bar, that thing was completely down)
So i asked support numerous times and they tried to bullshit me. Please send a video, there are no known issues, please delete your coolies, please sign out/in again only to post an service incident hours later.
I was not alone with my problems, other people also started to have problems, but they decided to play it down, then finally release an update and declare it to be solved only to solve it for 3 days on.
Worst part, the unavailability even stopped them from accepting mail, so they were dropped and lost… was embarrassing to call my customers by phone and ask them if they have sent an email.
It was so fkng stupid, i couldnt work, they didnt want to help and blamed me. The search bar was bs. The featureset for a company was too small and pricing got even higher before that incident (however I was allowed to stay on visionary plan)
Switched to a managed and hosted mailcow instance since then and never looked back!