I agree with the criticisms in this article. I currently have a 1.012 day streak on Duolingo, which I’m using to learn German, Esperanto, and Greek. I’ve “completed” the German course several times as the course was expanded, and even with the help of Clozemaster and Memrise I’m still only a B1 on an online CEFR test. It’s fine for the basics, but there’s no way it’ll make you good enough to, say, start working for a German-language software development company.
2000+ day streaker here, 5.5 years give or take, and I agree that the original article reads about right. I feel Duolingo hit the sweet spot roughly two years in, that is to say a good balance of introducing new vocab with revision of previous topics. It's still a very good way to pick up some basics, and I learned more with it than I ever did when I studied a language as part of my secondary education.
When Duolingo changed the way the skill tree worked a couple (?) of years ago it felt like it became more of a grind. I know they have some pretty smart people working on the app though, so I'm sure they're going in the right direction; but it feels like it's now easier to progress through the tree and the amount of vocab has been reduced, including hiding/simplifying the grammar study. IIRC in the original tree you had things like the "passé composé" introduced sooner, now they appear at a point that I suspect most users will have given up.
FWIW I tried Memrise early on and I didn't like the non-literal approach to the flash cards. I've been using Anki, which has some good decks but they can be questionable - for example the top 500 French words doesn't include the articles with the nouns, not a good thing.
Of course, the biggest things that have contributed to learning: moving to a French speaking country, switching all my devices to French, attending some classes, volunteering in a local gallery where I need to speak and use some vocab I wouldn't usually encounter. Still a long way to go though.
