I think it depends on what exact level you're currently at in your language learning. If you're a total beginner then I personally think that having a teacher

I think it depends on what exact level you're currently at in your language learning. If you're a total beginner then I personally think that having a teacher who can actually explain things is invaluable. If you don't have the basic rules of the language internalized and you don't have the fundamentals down you're going to have a harder time doing it on your own than someone who can just ask a teacher and immediately get an answer. I think the sweet-spot for language learning apps (at least for Japanese) is learning vocabulary and kanji. Those are two things where having a teacher doesn't really help all that much, and where an app can really help. But, again, a total beginner (which is the market that Duolingo targets since that's where the money is) doesn't need to grind vocabulary nor kanji; they need the fundamentals, which they won't really get from it.