User Kortaggio [0] talks about how Duolingo's claim “an average of 34 hours of Duolingo are equivalent to a full university semester of language education” is misleading because it's based on their long-discontinued SRS algorithm. Sounds like false advertising.
The gamification features are one-size-fits-all (whether you're age 13 or 30 or 65, occasional or dedicated user) and default-on: interstitial animations, success videos, chests of gems, then yet more 5/15/30-second ads... I challenge you to measure the % of time within a 10-minute segment that is actually spent on learning. Or at minimum, they could show me the cutesy stuff while I'm doing the language task, Super-Puzzle-Fighter style.
I'm a 3.5-year user (>1200 days), I clearly don't need somersaulting owls and faux-challenges to keep me going and I passed that point 1190 days ago, if not always.
Many (adult) DL users write about how the ever-increasing pushiness with leaderboards, quests, streaks ('create false urgency' per Candy Crush which the former CPO cites as a model) turn them off. Should have different use-modes for different segments of user.
I believe DL care about teaching effectively only up to a limit, and that limit is anything that might reduce their metrics, as a freemium app. They don't care about teaching Chinese effectively, not since back in 2021 if not earlier, they simply gave up. This is quite sad and weird for the world's second-largest language. There isn't "still lots to improve" on the Chinese course, they simply gave up and locked the forums permanently so that now not even well-intentioned volunteer users can fix that one.
Also, DL committing intentional trust and GDPR violations without any online opt-out doesn't sound ethical "Facebook Still Tracks People on Yelp, Duolingo, Indeed" [1]. Obviously tracking without user consent is for monetization of ads. Like I said, monetization trumps other considerations at DL.
(PS I signed up for your ReadLang, but for Tagalog it's pretty beta. Good luck with its second incarnation).