I temporarily moved from Fastmail to Proton Mail, but found several downsides:
* Loading mails (especially when going quickly through them) has quite some latency. Possibly due to decrypting.
* Search is pretty bad compared to Fastmail.
* The keyboard shortcuts seem to never work correctly.
So I went back to Fastmail for mail (we still use Proton Drive, which has its own set of warts).
Agreed, there are quirks, and there is also an AI writer, though it never disturbed me and hope it never will.
But generally the UI is clean and they align with my values.
There is a bridge software that bridges Proton and imap so you can use any client. Never used it though, I find the apps pretty good. The latest generation is focussing on off-line use which will hopefully drive down down some of the latency, especially that of the calendar app is pretty bad atm.
My father somehow loves the search, but I myself am also unimpressed tbh. I do like the tagging system.
+ I'll add the obligatory complaint about the lack of Linux Protondrive client.
I changed from Gmail to proton many years ago.
The only pain point I have is the search. Understandable, the emails are encrypted and search has to be done on the client. That works when using the web hi as you have the option to index all emails.
I do not find this option in the iOS app :(