Their calendar, contacts and bridge don’t support CalDAV/CardDAV, so you can’t synchronise them anywhere.

The iOS app doesn’t synchronise contacts or calendars either. There’s a one way “upload to proton”, but not the most helpful.

The public holidays only include some countries (not mine).

Their VPN is terrible with 20% packet loss, despite sitting in the same data center as other VPN providers without that issue.

But, still not google, and their mail app is better on iOS than fastmail.