Yep, I've got nothing but good things to say about Duolingo :-)
As a viable language-learning app (on the freemium tier), it's mixed and getting worse. I'm a 3.5-year Duolingo user.
See the 2/2023 HN post "How Duolingo reignited user growth" [0] describing how they/ the former CPO aggressively traded off reducing the utility for actually acquiring language vs too much gamification (not all users want 'Candy Crush' levels of gamification), user growth, leaderboards, challenges, cutesy animations, inventing silly revenue streams like 'streak protect' (all prior to the 7/2021 IPO, ticker 'DUOL').
Duolingo locked the Chinese course mid-2022 [1], that means no more fixes ever, no corrections, the broken or missing audio (and answers, and hints) stay broken or missing, all discussions (users helping users, handy links to third-party sites) are frozen. For no good reason. They don't give a hoot for gamifying valuable user contributions which add/improve course content/fix mistakes. If any of you were contemplating paying for DL Chinese before they locked it on the charitable basis that they'd fix the broken bits someday after they'd banked the IPO loot, well obviously not now. (Here's e.g. Langoly's list of "The 11 Best Apps to Learn Chinese (Mandarin) Fluently" [2])
By comparison the Spanish course is alive and being improved; they added Stories.
TL;DR: optimizing the product to juice up metrics like DAU prior to the IPO is good for valuations, but is not a great long-term approach to a language learning app and its community.
[0]: https://news.ycombinator.com/item?id=34977435
[1]: https://www.reddit.com/r/duolingo/comments/zjnwmh/why_is_chi...
Obviously Duolingo isn't perfect but they're very aware that there's still lots to improve. And sure, maybe their approach to gamification isn't for everyone, and that's fine, since there are many people who do like it!
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).