@julian yes yes yes yes!
Years ago when i first looked at NodeBB, I was thinking about it for a discussion-focused social network (kind of like the old tribe.net, or Facebook/Myspace groups), and this was a key area where forum software in general didn't meet my needs -- and it wasn't obvious how to extend it. So I rolled my own as a prototype and it's stayed as a prototype for a dozen years because trying to do a full-fledged implementation that's maintainable and scalable requires implementing a forum system, decidedly non-trivial.
@julian yeah really. in my defence I knew it was hard to do anything scalable. The prototype was good enough to explore several potential directions but the answer kept being that they could potentially work as customizations/extensions of a an existing (scalable, accessible, secure, highly customizable, very flexible) underlying platform with great moderation support, but that didn't exist yet and wow it was not the kind of thing i could just code up on my own.
I do think that for this to work the support for moderation has to be quite good both at the per-group level and at the site level. Community moderation and per-group moderation, if done well, can help a lot with the scalability. Still, site-wide moderators still have to deal with rogue and abandoned groups, and situations where group-level moderation leaves people at risk).
Of course you actually need site-wide moderation to deal with most of that stuff things if the site admin creates all the categories. And user-created categories/forums/whatever are something that would be really useful in a lot of differnet use cases, so IMHO really worth focusing on!