Meh, when you say Duolingo isn't great, you should compare it to something in its same class. I don't think Duolingo competes with sitting down with a grammar book. The whole reason I used Duolingo, which got me up to speed with Spanish enough to read books, is that I could do it anywhere and any time rather than bust out a book for serious study. I was never going to do that, so Duolingo was strictly better for learning a language than a book.
People lose track of that every time they dunk on Duolingo.
There are better apps for that than Duolingo though, it does not have a monopoly on app based learning of languages. For example, Anki, Babble, Memrise, etc. This isn't to even mention simply reading a book like the one I mentioned, just on your phone instead. That is in fact how I read most of that book. There was also an audiobook version which I used from time to time to get the pronunciation right as well. My point is that studies have shown that Duolingo does not measurably increase language learning for most people, most of the time, it is more akin to those brain or puzzle game apps that people use, which also have been shown not to actually increase any sort of learning.
People lose track of that every time they dunk on Duolingo.