I use thunderbird since years and i'm fully happy with it.
Now i had a customer, which currently uses a webmail solution with a "deskapp" wrapper. Currently plan to move away from it and the first idea was to use thunderbird with a caldav backend (nextcloud with tbsync) .
The only issue which some users mentioned was, that they currently have a calendar view to see the calendar paralel and not merged in one view (with different colors). So that he can set the "day view" and see each persons calendar side by side. To view which person has on which time a free slot is then better when you have around 15 people (remember the color of each is not so easy for all). Outlook has also this feature when viewing calender. (arrow to left and right on a calendar) so i added one from outlook here (german version).
I searched but don't find any feature request relatet to it. First i think, perhaps someone write an addon, but don't find any 😞 I't woul'd be greate to have this feature.