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...