Maybe it does depend on the language, for Spanish my native Spanish speaking girlfriend will often correct me and sometimes say what it is teaching me is just flatout wrong.

On top of that it is teaching me South American Spanish which I guess would be fine if it told me that was the case but it doesn't make a distinction, ever. I want to learn Spanish proper, something like Busuu will take the time to tell you "in this region these words will be used where as in this other region these may be used more commonly"

Additionally its actual method of teaching is just like throwing shit at a wall and seeing what sticks, it doesn't actually explain concepts properly to you, it doesn't explain why things are happening within the language. Add on top of that the shitty heart system where you are penalised for not knowing things by being looked out of further learning and it just isn't an effective teaching tool at all in my experience.

I have learnt more in a month on Busuu than I have in 550 days of Duolingo because it explains things properly and actually tries to teach you the language rather than just throwing words at you until you get things wrong and get locked out for a day.