It's not useless at all but the quality of the courses varies. I've been doing the English to Spanish course (the most developed course on there), I'm on Section 6 (Upper B1) and I can now read the majority of Spanish subtitles in real time. Duolingo has been my main resource for learning by far, the only other thing I've done is watch some Spanish shows on Netflix and Youtube using LanguageReactor as well as subscribing to some Spanish subreddits and using DeepL for translation of words/sentences on there that I haven't encountered. Once I have completed Section 8 (Upper B2) I plan to do the iTalki language test[1] to find out what I need to work on and will be preparing to take one of the official DELE/SIELE/CELU qualifications at B2 level with a view to moving to Spain for study/work.

To get the most out of it, I would recommend reading the built in grammar sections multiple times as well as subscribing to language learning subreddits and googling things you don't understand. I suspect this will become less of an issue as they roll out their AI offering which bakes this into the app (e.g you can specifically talk to the AI and delve into the subtleties of the grammar).

I don't know but I suspect they are focussing on perfecting the most popular courses and getting them compliant with the official educational framework levels and then using those as templates to flesh out the other courses. They've also explicitly stated they're looking into AI to auto generate content as at the minute every lesson is handmade and approved by humans. I suspect once they've got this down, they'll then expand the Duolingo Language Test to more languages other than English.

[1]https://www.italki.com/languageassessment/ila