+ Calendar sharing in the organization is opt-in, so it can force more coordination to make a meeting. Maybe that's a good thing?
+ Other peoples' calendars often appear in the same color as mine, so if I have somebody else's calendar in my view I might think I've been invited to a ton of meetings.
+ Email contacts are weird in a way that I can't quantify. With gmail, when I hired somebody, I often ended up with both their personal email and their work email in my address book. Gmail knew to default to the work email after they were hired. Fastmail does the opposite so I have to remember to delete my new employees' personal emails from my contacts so that I don't accidentally send invites to the wrong address.
All that said keyboard control of fastmail is cooler than gmail and the rules are really fun to work with. I like that you can use sieve rules with it. In fact the lack of tight integration makes it easier to use different services for different things. No longer do I have to unselect "make this a google meet" link or whatever that was.