The thing to understand about Duolingo is that its weird and unhelpful sentences aren't meant to be useful sentences. Since Duolingo isn't a phrasebook, these sentences serve the purpose of drilling you on constructions and creating new phrases from the get-go, without going through a mountain of texts like textbooks would have you do. In German, they might teach you "ich bin ein Apfel," along with other nouns such as "Mann," "Mädchen," etc. Well, now you know how to say 3 sentences for the price of one (ich bin ein Apfel, ich bin ein Man, ich bin ein Mädchen). These drills take a while, though, and that can be a problem as Duolingo might be the most mind-numbing experience for you if you dislike the medium. Really, the most difficult language you can learn is one that isn't fun for you (applies to language apps and how you learn the language, as well).
That said, Duolingo itself isn't something that works all on its own. Really, it's a supplement. Usually you can pair Duolingo with a textbook or course in school for some extra vocabulary. If it were me, though, I'd just use Anki for that extra vocab and grammar. Some people have trouble with Anki's UI being so sterile. The issue with most language apps nowadays is that the "good" ones all cost money. Duolingo is free, but now paywalls its decent features which were the only features distinguishing it from just a flashcards app with fancier graphics. Every other decent app with innovative features won't allow you to learn more than 5 vocabulary words without saying "oops, that's enough language to you, give me money, now." That's a different rant, but using an app on a subscription service just isn't a good idea unless you want your money drained. A better one-time purchase is a text-book, such as Deutsch Aktuell.