I completely agree on your first point!

I agree with your second point that Duolingo is a helpful tool. My issue is I've seen people rely only on Duolingo, then become frustrated when they realize they haven't made progress.

It's a good app for basics. But in terms of learning grammar and achieving fluency, there are not only different apps to learn that, but also different methods.

A lot of it depends on your first point. Some people binge Duolingo like they do a TV show and somehow expect themselves to absorb the content when that's not how it works.

Which languages did you learn by the way?