Duolingo works reasonably well when used as a supplementary aid to learning a language. I've found that it has reasonable utility when combined with taking a physical class as a low-effort boredom filler for building familiarity with the vocabulary of a language.
What I find pressingly missing is any meaningful way of learning about the grammar of a language. When learning German there was absolutely nothing in the lessons which even hinted at the rules of tenses, cases, conjugations etc and the lack of that content stunts the possibility of building much more than a rote-learning knowledge of a language.
The app does seem to have a great gamification mechanic and is good at driving user engagement, so I guess they have that though.
The web version of Duolingo used to have detailed explanations for each lesson that explained a lot of grammar, they seems to have disappeared recently
What I find pressingly missing is any meaningful way of learning about the grammar of a language. When learning German there was absolutely nothing in the lessons which even hinted at the rules of tenses, cases, conjugations etc and the lack of that content stunts the possibility of building much more than a rote-learning knowledge of a language.
The app does seem to have a great gamification mechanic and is good at driving user engagement, so I guess they have that though.