Something I think that the PIP categorised points system doesn't properly allow for is forms of disability arising from overlap between more than one impairment. You could have two impairments, neither of which is worth points, but the intersectionality of which is definitely worth points.
Also, it's bad enough that in various fields of medicine there are prejudices and stereotypes, but those will, I believe, inevitably come through in benefits assessments as well.
I could rant forever about these issues, so count yourselves lucky that I'm restraining myself!