It's interesting to see how many people don't find Duolingo or other apps to be effective learning tools. I spent the last 2 years creating a Thai learning app (that I think of as a 'digital textbook') specifically to address the issues that many people here raise. I won't cite the name since I'm not trying to advertise, just bringing it up as a talking point.
Duolingo does lots of AB testing and found that gamification greatly increased user engagement, but I think that they took it too far and lost the forest in the trees.
Everyone learns differently but in my experience you need to hear or read explanations of the second language in your native tongue and then hear that construction in the second language to reinforce it, because the key is lots of comprehensible input.
Can a complex sentence become comprehensible input with just games? Sure, but if someone can explain how the grammar works in English then you can get to that same point faster and without the guess work. That's my opinion, at least, and so far users are seeming to find it effective since reviews have been largely positive!
I think that serious language students will seek out appropriate apps or textbooks, and Duolingo will be a fun option for people that want a mixture of cognitive exercise and entertainment that may also be useful when they're traveling or interacting in a foreign language.