E-mail notifications in calendar
The current ProtonCalendar that's in beta has one fatal flaw: no e-mail notifications.
On the desktop, that means that you need to have ProtonCalendar open at all times, as without it, you can't receive notifications.
On mobile, that means you basically never receive notifications, as there are no mobile apps (yet).
For these reasons, the lack of email notifications makes ProtonCalendar unusable for me, as I need to be reminded and informed about my events.
Scott Hammack commented
I agree; this is the biggest thing keeping me using Google Calendar rather than ProtonCalendar.