Few limitations I can see for now:

• in contrary to issues, there are no automatic generated back-links when mentioning another discussion with #discusion_number between discussions or between discussions and issues. This does not help finding relevant information in related discussions or issues. Links between Tickets and Discussions are not shown (in the opposite direction) community/community#3211 (comment)
• no templates for Discussions: Templates (like issue templates) for discussions community/community#2838. Looks like this has been partially implemented see https://github.blog/news-insights/product-news/github-discussions-just-got-better-with-category-forms/

Old pain points that have been fixed since this post was created:

• move a discussion to an issue is not possible although it could be useful. edit: this has been implemented (Convert Discussion into Issue community/community#2861). An example use case : a question started in Discussions lead to realise there is a bug in scikit-learn. Currently you have to create a separate issue with a link to the Discussion.
• for categories for which there is not an answered mark (currently all but Q&A), I feel like sometimes you want to say this discussions has reached its term and then maybe not see it in any more the main dashboard. Does that mean there should be a way of "closing" a discussion, not sure (Close discussions community/community#3097) ... you can lock discussions but this seems a bit harsh ... you can close discussions now https://github.blog/changelog/2023-03-14-adding-the-ability-to-close-github-discussions/
• • no labels yet edit: this has been implemented (Provide an equivalent to "labels" community/community#19). For example some users may be knowledgeable and willing to help on a particular topic (e.g. gaussian processes). Filtering by label on top of Categories could be useful for them to find relevant discussions.
• bug about triage rights through a team When triage rights are granted through a team, the "Convert to discussion" button does not appear community/community#2993. We don't really use triage rights anymore

In general, I think scikit-learn/Discussions must become a place of a clear purpose and usefulness. At the moment, Discussions seems like everything and nothing (also barely alive). In my opinion, the best way to define the purpose of Discussions is to list Discussions among the mailing list, StackOverflow, stats.SE, and gitter, which will help to ensure that the scopes of these channels of communication do not overlap too much.

In particular, I'd like Discussions to become the place for aspiring or beginner contributors to seek answers from maintainers and core developers, as Developer's Guide cannot document all aspects. The kind of questions I asked, I believe, is more suitable here than on StackOverflow (these questions are specific to the current scikit-learn CI setup and development practices). Also, answers are found much more easily on a discussion board or forum than in a chat room or mailing list (I could not find the search function on gitter at all!).

I suggest to create a separate category: "Q&A for new contributors", and give it a year or so to see if it takes off.