I think the challenge is that it looks like a lot of other 'standards' I've seen: on one hand tediously specific yet on the other hand, so open ended as to largely defeat the point.

Every problem must have a 'type'. Ok, fine, so what are the semantics of the 'problem type'? Well, nothing in particular, just has to be defined, but it might be nice if it's a url telling a human about your own human thoughts on the type. Also, if you encounter multiple 'errors', you need to omit any that you arbitrarily fail to group into the same 'type' which shouldn't be subjectively too vague either, so don't think about making catch-all types so you don't have to discard some of the errors.

You can't count on the members, and a problem type may arbitrarily 'extend' to completely rearrange those members into members of child objects instead, but that's really all up to the backend to decide however they want to arrange it, with no prescribed standard for error bundling, but an example of how a backend could voluntarily implement such bundling as an implementer specific extension if they like, but again, don't bundle errors that shouldn't seem to be of a common type...

Also I think it's funny that they say do this in the name of being a good web citizen, but then say send this new mime type down, client's Accept header value be damned.

It purports to drive toward "machine-readable" problems but it seems like there's not much actually actionable and the client has to in practical terms do a bunch of bespoke handling to deal with a backend that is still pretty much open to do whatever they like.

It has a couple of reasonable seeming examples, but nothing that would make me think "Oh, you implement RFC 9457? Then I already have error handling code ready to go!"

I've seen all sorts of complex errors generated by backends that have all sorts of features like this and more (error messages with parameterized string values, json pointers to specific problematic pieces of the client request. However people just want a human readable response to pass along. I could imagine the example 'pointer' being useful to map error details to a client maintained form, but that's not even the 'standard', just a random example 'extension'...

Successful web request, web server functioning fine, application functioning fine, but the application needs to tell the user they have done something wrong in-app.

In my world for example there's a piece of third party hardware I need to take a bunch of data from and deliver it to the user. One part of that data is telling the user there's an error and they need to perform an IRL interaction with it to fix something, but the rest of the data and images in the payload are good