Duolingo used to be a very effective gamified language learning system.
Then they decided they cared more about profits than providing a quality product.
Now they are best known for their dark patterns.
Then they decided they cared more about profits than providing a quality product.
Now they are best known for their dark patterns.
https://research.duolingo.com/papers/yancey.kdd20.pdf
I assume all the research after this point is too revealing to publish.