When playing through the campaign, I found that the UI held more surprises than the tactical battles. It's mostly small things that make using it quite cumbersome.
A double-click on a city should rather open the "details" screen than build the first available building. (The latter really is the last thing I want to do without even looking.) With the "details" screen open, I would like to be able to remove things from the build queue. And, above all, it want to open the "build" or "train" tab with one mouse-click. For some strange reason, the "details" screen is not a tab which is connected to the city panel in the bottom left corner. The "build" and "train" tabs are much better. Well, maybe they could be merged.
EDIT: Sometimes the city view (90° perspective) does not slide back to the normal view (45° perspective). Triggers seem to be: (1) Ending the city view with ESC, or (2) Using the arrow keys to move to another city and then clicking "done".
Can I see somewhere which groups/parties of units still await orders? "Pass" (on the strategic map) does not seem to have any effect. (EDIT: Something more obvious than the green frame should be used, like the "ZZZ" for idle cities, and the green frame should be reserved for the currenly selected unit.) "Explore" is not easy to cancel when the unit(s) is/are sent back to normal duties.
Switching between "equip" and "shop/trade" is not easy (the same as with buildings). (Most of "equip" can be done on the "shop"/"trade" panel - but not "use".) "Shop"/"trade" mostly ignores selected units within a group. I would expect tabs in the bottom left corner, not buttons on the top right corner, when switching dialogs.
It is not easy to predict which units will be affected by a command. Some tactical battles only take eight units out of a group. When units are moved out of a city, the group size is much more constrained and it is not clear which units will fit in. Sometimes (when "equip" or "trade" was used) the champions move separate from the rest of their parties. In short, it should be easy to reorder the units in a group and move units between groups on the same tile. Maybe there could be global groups assignable to number keys 1, ..., 0?
And a minor last thing: Can I see which rings and other items take up two slots before I purchase them? (Why would a ring do that anyway?) Why can I equip, for example, a compass and a scar stone at the same time although either of them cannot be combined with other items?
That's all which comes to my mind at the moment. Thank you for listening.