I used Duolingo for about 1 month to learn Portuguese and was close to the end of the tree. I learned quite a lot, the grammar drills were useful introduction and basic vocabulary, however I stopped using it due to it requiring me to translate back into English which made it hard to think in the language. I bought other books, like side by side texts, and grammar books to continue and watch YouTube videos from Brazil to drill in the sound of the language.

I learned German at a University in a German speaking country, and the classes were completely in German from day one which I found helpful. I also bought quite a number of books and studied quite a lot before I become fluent.

So I don’t think an app is a one stop solution, it takes a lot of material, I’m not sure why people think any one system is enough, especially considering you never stop learning a language. Fluent just means you know enough to continue to learn in the language on your own.