I hate that I can't view the calendar separately from the email editor... it is convenient to have the integration in terms of invites/notifications and the availability for scheduling meetings is nice.

My biggest gripe is the distributed db that backs exchange/outlook365 mail on their server... I operate on inbox-zero and when I clear all the filtered mail out, then try to empty the "trash" I either have to wait 2 minutes for all the other deletes to sync to the trash, or empty the trash multiple times.

It's significantly worse than it was in the 90's on dedicated server(s), though back then the hardware was really easy to overwhelm with too much email/activity.