The only thing something like Duolingo could be good at is learning vocabulary. But I found it pretty bad at that. I had much more success with Memrise. However, this was many years ago and last I checked Memrise had regressed significantly, at least for me.

Duolingo is a terrible way of learning languages overall. Almost as bad as Rosetta Stone. But, like Rosetta Stone and other things like gym memberships, it doesn't have to be effective to be profitable.