This one is to save you a lot of time. I am unsure, how many people are still to upgrade but sharing anyway.
Issue: When you upgrade from web client to UCI, some columns in the views will show as blank.
Reason: I couldn’t find any reason and then Microsoft came to rescue. This might occur because the missing data column is not in the FetchXML of the view. UCI depends on the view metadata to retrieve data from server. In web client this was different.
Solution: Remove column and add it back, save and publish. And the data will start appearing again. Make sure to do this in dev/test/sandbox environment first and then move the solution to production.
Hope that helps!