Does the app work? I don’t mean the gamification: I mean, are you becoming fluent (or even just literate) in the language outside of the app?
You _will_ need to supplement the vocabulary with Anki or another tool. You won't become fluent, but you will get a good basis to start talking with people comfortably. Becoming fluent depends on keeping up that talking. I would say that Duolingo gives a terrific starting point, at least for the three languages I've learned with it.
From my point of view, you can make it to A1/A2 proficiency in a language with Duolingo alone - after that, it gets quite hard. Teaching grammar and conjugations isn't its forté, without resources outside of Duolingo it will get rather tricky.
That being said, I still use Duolingo for my daily dose of language exposure - but without Anki (for vocabulary), youtube videos (to explain various specific topics) and evening courses (which help a lot by giving a structured approach on what to learn in what order), my learning curve would probably flatten out.
After nearly a year, I can help set up for dinner ("Where are the plates?" "There is bread on the table already" "Do you want tea?") and similar things.
Now I'm getting into genitives and such, and that's where Duolingo really seems to not do well. It doesn't explain why a word appears a certain way in a tense, it just makes you memorize that it does. Sometimes I'll ask my husband why, and he just says "Well, because that's the way it is". Of course I'll use the same explanation when my MIL asks me something about English.
Most native speakers are useless for why. You ideally want someone who learned both your native language and the language in question. Then they can often explain things relative to your language, and you'll learn things about your language and the language in question. Look for language notes from Hong Kong; I had found a university site with very helpful information on French many moons ago, but I can't find it again, it was written in fluent English, but with insights on English usage that were unlikely to occur to native English-only speakers.
I did French all through high school and most of college, then learned firsthand that I was maybe 25% as fluent as I assumed. Then I went home and went through the entirety of the Duolingo curriculum in a few months, without learning anything new (a couple new vocab words, maybe).
So, you could view this in two ways: either it's a scam that tricks you into wasting a lot of time viewing ads or paying a subscription without delivering on its promise. Or, you could see it as a shortcut that's worth at least a couple years of classes. But, at least in my experience, it does not enable you to speak a language with fluency.
(Which makes total sense if you step back and look past the marketing promises. Of course 20 minutes a day with a cartoon bird is not going to replace actually living in a country and being immersed in the language 24/7)
key problems:
1. not enough speaking practice. speaking is _at least_ 50% of the skill but about 10% of DL.
2. Waay too much time spent setting up each question w animations etc. not enoimugh learning time. much more repetition is required.
3. way too general learning. people have varying needs for language. e.g. work, travel, family, research/reading. teaching must be tailored to these.
Else you're like a dog listening to steam of human sounds: "Not walk, not walk, not walk, ...WALK!! (I know that one!), not walk, not walk, ..."
In my experience, Duolingo and other entry level apps can get you there especially for the person who hasn't learned a second language thus doesn't even know where to begin.
Not in my experience.
Anki + language-partner sessions + evening classes did it for me.
Luckily, that is a lot easier these days with the internet!
I can still understand a lot of Spanish and when I’m warmed up and have been back a few days my conversational level comes back.