Three years ago, I casted my doubt on the English learning method at Duolingo for Chinese.
I posted my questions on
- Zhihu: 在Duolingo通过中文学习英文效率真的高么？ - 知乎
- Duolingo Community: Learn Spanish, French and other languages for free
- and my own blog: Is it kidding to use Duolingo to learn English from Chinese? - Sinostep