Functionality of iOS app
First at all: given that it is entirely free both of charges and ads, Duolingo is amazing. So the following point is no criticism, rather a suggestion.
Both on the web and android version one can redo the test out / skill strengthening of single categories even though the skill bar of that category is still full. In the old iOS design, this was also possible. At least for me this functionality was really useful as I like to redo strengthening of single categories e.g. immediately after having passed the lessons. However, this seems to have gotten lost with the introduction of the new (otherwise really cool) iOS design. Now I can only repeat single lessons or do the general "random pick out of everything learned so far" strengthening.
Hence I believe that restoring some "repeat an entire category independent of strength levels" functionality on iOS devices would be helpful.