I do know of these cases, of course, but the field of mental illness is very much different today than what it was in the days of "Hyesteria" - both with regards to diagnostics, and to research. For instance, there are well known structural and functional abnormalities associated with certain mental illnesses - which would seem to imply that the current categorizations aren't completely off.

While you may be right in the technical sense that future research can make our current understanding obsolete, I don't think there's any doubt that some people suffer as a consequence of what is currently conceptualised as a behavioural disorder called ADHD.