Imo all these complaints against duolingo not being good enough are not fair. Have you seen the alternatives ? I mean they did get 1 billion dollars for not nothing !
I have used several of the alternatives, and all of them are more effective at teaching languages. Granted, Duolingo is better at raising money.
If Duolingo were as effective as sites that require the user to make actual effort, its popularity would be more than justified.