Right now, Proton has good-enough email and calendar tools, but assumes everyone lives in an isolated world.
How many of us have existing Exchange/O365, GSuite etc and need to be able to see things like all our calendars as a single view of the day.
Right now I can't switch to Proton Calendar because I can't see what's in my work calendar alongside my family calendar, and I need to deal with two mail clients etc.
Even having someone send me a Google or Outlook calendar invite doesn't work particularly well, - in the Android app, I can see the invite in mail, but can't accept unless I go to the calendar, search for the event, and accept it (iPad and macOS app at least give me accept option), but then I can't see it in my Android or iOS calendar widgets.
If the original meeting is then cancelled, I just get a blank message.

the whole point of a connected internet is stuff should work together. I'm not saying it's easy, but with a renewal coming up and lots of rough edges in mail, calendar, and pass are really making me wonder if the price tag is worth it