Consensus among Serious language learners (i.e aiming for a B2+ level) is that duolingo is marginally useful for learning some vocabulary. A thorough course or textbook are essential
Going to a fixed sequence of learning "the path" so that you're forced to cover topics and words in a specific order, and then introducing grammar that has never been taught... Why? Either cover them in order, or don't.
Eventually, you figure out the right answer by trial and error and "learn" it, but only after having developed a bad habits and wrong answers... Why? Of course there are Anki's and such out there that cover all the words so I guess they want you to just do that.