My two cents: I tried learning a bit of German through duolingo in the past and I agree, it's completely useless.

Recently I started taking Spanish classes and it's nice. Classes teach me grammar and a relatively small set of words, duolingo is teaching a few more words.

The amount of advertising is too much imho, and the paid subscription is too expensive (as in, not worth what I'd be getting).

So overall... Yeah it's a bit weird that duolingo as a company stays afloat at all.