Using the rwmb_frontend_validate filter we can prevent the post from getting created by returning false which is really useful.
It would be really nice on top of that, to be able to add some feedback/errors somehow at this point that are displayed on the frontend. Right now, even if the post isn't created (rwmb_frontend_validate filter result returns false) there is still a confirmation. If that could display feedback as to why the post wasn't created, it would be even more useful for server side validation.
A good use case for this might be server side validation of file uploads - ensuring the file in $_FILES is of an allowed mime type and file size. Client side validation of this can always be circumvented.