Usually adults have some kind of extrinsic motivation for learning a language: to use it while travelling, to become proficient so they can live in a certain country, to talk to their boyfriend/girlfriend, to use it at work, in other words to communicate with other people in some set of situations. Duolingo may or may not help towards their goal of communicating with others in the situations they desire.

If you just want to study a language for the hell of it, without a goal in mind, then Duolingo is fine. Adults tend to fail at learning languages with only intrinsic motivation because it’s a huge time sink.