Well, true to some extent...
However, when an "expert witness" is to testify, they have to have vetted credentials to do so. I guess it's a bit of "pre-screening" which amounts to calling into question the reliability of the expert's expertise. I'm not sure if it's entirely relevant here, but ...if we're making psychological evaluations, the psychological knowledge and experience of a person can come under question. And since psychology is "about the mind", I would imagine reasonable sanity is a bit of a prerequisite. Otherwise, we'd start having the lunatics running the asylum. I'm sure they'd at least be proven NOT to have whatever disorder they are being called to weigh in on.