The biggest problem with Duolingo is not just the spaced repetition but the entire curriculum is random, imprecise and felt like slop long before LLMs (with the exception of stories I guess). By random I mean it's just like reading a series of disconnected sentences with some common features. There's no anchor.
For Spanish for example, compared to spanishdict.com course which is a similar format but has a defined length and doesn't encourage you to continue indefinitely, is much more precise, follows a practical story arc - introducing yourself at school, how did you get to school, going on vacation with family etc etc. It jumps from location to location each unit and explains regional differences in grammar/vocab.
For Spanish for example, compared to spanishdict.com course which is a similar format but has a defined length and doesn't encourage you to continue indefinitely, is much more precise, follows a practical story arc - introducing yourself at school, how did you get to school, going on vacation with family etc etc. It jumps from location to location each unit and explains regional differences in grammar/vocab.