It's so fucking lousy with gems and upsells and quests and "try the AI call, you get one for free and then you can upgrade to a more expensive subscription!". The gamification of everything in Duolingo is so bad. And, as you say: I feel like I'm wasn't learning any of the fundamentals, can't you just tell me how to conjugate this verb and then testing me instead of making a dumb guessing game out of it?
Compare it to (say) YouTube Premium: I know a lot of people hate it, and that it's an expensive subscription, but I honestly think it's pretty fair. All the commercialization and upsell go away entirely, you get a very clean experience for something that I use more than any other streaming service. Don't mind paying for it at all. Not Duolingo though, I cancelled it after a week and a half.
They would be seen as a stuffy edtech companion app if they did the unsexy work of teaching. Gamification = user growth and engagement = investor money.