The biggest issue I had with duolingo when learning Norwegian was at the beginning and when introducing new concepts like definite articles.
The fucking app for some reason does not have tips (well they do now, but they are not really tips just the lesson vocab) but the website has them and they really are way better at explaining how some things work or are formed. The lesson just throws words at you and expects you to figure out how the concept works completely on your own.
I had so many mistakes when I was learning the definite article because some words were only ever given in the definite form and I had no idea and just thought that is how the words were.
The system works alright when you get the grammar and the basic langiage structure and the lessons basically just add more vocabulary. But the beginning of a course is unstructured and sucks absolute ass without any real explanations. Maybe more popular languages are more structured but that has just been my experience and I have a 1320 day streak currently