That's too dismissive in my view. Duolingo isn't as good as in-person lessons, but it's still way better than nothing. I'd also argue that grammar exercises are cripplingly boring for most people, so if the choice is not learning or grammar exercises they'd choose not learning, which is worse. I'd also argue that you're only talking strictly about learning language in the most efficient way, but missing out on the fact that if you can make it fun, the enjoyment in itself is a valid reason for doing something.
Edit: yes, I am dismissive because I've tried it and seen that its obviously a scam, in the sense that it's sold as a fun way of learning but it's actually just a fun way of not learning.
