My anecdata is that both Duolingo and LLMs are useful in learning. Yes, you have to put in the work.

Seeing the usual HN hate on Duolingo specifically, I wonder whether it's blind. I might have missed scientific research about it causing cancer.

Responding with my own anecdata: Duolingo is not bad, but better options exist, in with a gap in quality and quantity that you might as well just use those (admittedly, I have been learning Spanish and Japanese, languages that have a lot more resources than Mongolian).

Speaking with anyone who has intensively learnt languages, FSRS is the best way to remember vocabulary (you will need a separate source to actually learn it). And I've found that books often offer more structured practice and introduction. After you've outgrown books, picking up words from real conversations / media is much faster and makes you more likely to actually remember them.

And for Duolingo, there are many things to nitpick about. The obsessive gamification, the hearts (and the current energy) system that prioritizes getting answers correct by guesswork and cheating or being limited to a few minutes a day, the company firing chunks of it's core translation and language staff.

Duolingo is more than adequate, however, if you plan to do 5 min / day and just want to keep a streak going. I have seen people who have gotten real results from it too.