It seems it improved, but almost all Duolingo German students would eventually ask a question that Duolingo wouldn't answer. (They wouldn't understand genders and cases)

But the article touches the main core of the issue with Duolingo:

"You’re at the airport outside Rome, she said, and you want to get downtown; how would you ask? I gaped like a fish. ... “Do you have a table for four?” “I’d like two glasses of red wine.” I knew I had seen all the pieces in Duolingo’s sentences. But I was utterly unable to recall them and pull them together."