The thing that sorta gets me about Duolingo: If it became mainstream for everyone to do what is essentially 5 minutes of anki every day (which is kinda the Duolingo pitch), language learning would be kind of a bad candidate. If you spend 2 years memorizing 400 words you still aren't close to knowing a language.
But there are many situations where memorizing 400 distinct things is pretty useful: countries, capitals, recipes, history etc.