Duolingo user here with a 4 year streak.
Duolingo is not a language teaching platform at its core. It’s a gaming platform with language as its gaming skill.
Duolingo at some point became so focused on gamification that it just became a game (I believe they hired their lead PM from Zynga).
If you’re on free version, just look at the ads you’re getting. Vast majority of the ads are for other games.
I think you can learn a language if you use Duolingo’s streak gamification as a daily motivator but use supplemental materials to actually learn.