There's a bunch of us at CalConnect in Karlsruhe right now working on improving calendar standards :)

How do you use their calendar? Do they have a special app or are you talking about the calendar integrated in mail app?

Yes, I use their calendar web app on Desktop (the one next to Mail). For mobile, I sync individual calendars into my Android Calendar ("CalDAV-Sync").

My biggest issue with the Google Calendar was the syncing rate of 24 hours for iCal feeds. On Fastmail, new events appear quite fast (and I can force the update manually, if I need to).

I wanted to create a group calendar between myself and my wife. We both use FastMail. Gave up after fighting for a day with weird errors.

FastMail sent me an email that said our calendars are not in sync anymore.. way too often.

This seems like a really common use case that ought to work well. I switched to Fastmail a few months ago (I still haven't fully committed to sticking with it.)

Did you contact support?

Did you solve the problem by switching to another calendar provider?

I use their calendar with various apps like Fantastical or Timepage. It's standard CalDAV and should work with any decent calendar app, including defaults like Apple calendar.app.