My only criticism of Duolingo is the monetization. The ads on the free version are unbearable and makes me want to never use the app again. Also as soon as you stop using the app, they spam your inbox with emails every 2 days such as "Duo misses you so much! <sad emoji> Are you giving up on learning?? <crying emoji>". This made me uninstall the app.
Otherwise I don't share the criticisms in this comment section. It's a fun educational game to learn a few words in other languages. I don't think it's misleading anyone into thinking they will become polyglot by using Duolingo. I wanted to learn a few words of Japanese while waiting for a flight to Tokyo and it did the job.
It's a shame their original business model didn't work out. If I remember right as you learnt the language you could translate real web pages, and companies would pay Duolingo for the crowdsourced translation work. Win win really for the users. I think the guy that came up with captcha started it (similar idea).
Otherwise I don't share the criticisms in this comment section. It's a fun educational game to learn a few words in other languages. I don't think it's misleading anyone into thinking they will become polyglot by using Duolingo. I wanted to learn a few words of Japanese while waiting for a flight to Tokyo and it did the job.