Both of my parents are teachers of a European language. They both have phd's in linguistics, and rate very highly with students (who basically adore them).
All of this context to say that not once has anyone using Duolingo been able to "test out" of the first ("101") class that they teach. Duolingo self-learners come in with a very unequal mix of vocabulary and... not much else. Unable to use declension properly [0], unaware of most rules around gender, verb tenses, etc.
I'm sure (and I should look it up) that there have been academic papers written on these quite different methods/approaches: gamified learning vs "academic" learning, immersion by moving to a country, etc.
But in my parents' experience of teaching (which spans ~40 yrs), Duolingo students pretty much all became disappointed in the app: these students thought that they had developed skills when it turns out they mostly got addicted to a game that overpromised useful learning over entertainment.
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Imho, the ugly truth is that language learning as an adult is deeply hard and requires a tremendous amount of effort and "tricks" to keep yourself motivated. People who watch native media with subtitles, play with AI apps (such as the YC backed https://www.issen.com/ which is quite nice), take a mix of "classic" classes, spend time in a country where the language is spoken and force themselves into situations where they "have" to speak, etc. all do much better. But it's a ton of effort.
[0] https://en.wikipedia.org/wiki/Declension