Like many things, circumstances will differ.
One thing that I deem a falacy though, is comparing it with e.g. an expensive paid for classroom or personal lessons. It may well fail in such direct comparison, and no surprise!
People then use that comparison to claim "it's worthless", and I disagree with that.
If the claim is "30min of structured learning with duolingo will get you no closer to learning a language than 30 min of mindlessly scrolling facebook cat videos", I don't believe that's true. And THAT is the correct comparison :)
One thing to note though, is how you use it - note I used the term "structured learning" in previous sentence, not "doing match madness for points" :)
Personally, I spent first few months learning on duolingo, and it was wonderful; had some small chats with people who speak that language at work, occasionally wrote a sentence or two in that language in emails, etc. Then I spent next few months preserving the streak. I did the challenges and matches and got thousands of points and first places... but learned nothing. In fact, my knowledge deteriorated.
Then after 200 days, I lost my streak and... breathed a sigh of relief. I am now making sure I do the lessons and practice and learn, and purposely avoid the competition part of it or activities which give many repetitive points but don't increase my knowledge. My familiarity with language is increasing again and I can make more and more sentences on my own.