That's interesting to me. From my perspective, I didn't find Duolingo great, but it did give me some vocab and basic sentences, and left me feeling more competent than I actually ended up being once I was living where they speak the language I was learning.

Since then I did classes on-again, off-again and I can really feel my ability ramping up when I'm doing them, to the point where I was having short conversations in that second language. When I'm not doing classes, I'm still reinforcing things through my surroundings but I definitely feel that I plateau and don't really get much better.

However, the classes did get me to a point where now I can do things like play D&D in my second language. I still don't feel fluent (I have to active-listen the whole time which is tiring, and sometimes mentally translate still, though that's improving) but I am pretty conversational, and the classes definitely made a big difference for me.

Perhaps it's that there are classes and then there are classes, and you've had bad luck with the quality or nature of yours?