Broadly speaking, I've had mixed results with psychological testing. Mostly it's been useful in adults to identify intellectual disability diagnosis that was missed for whatever reason in childhood (often good verbal skills and growing up in a small town where everyone just got passed in school). However, I've seen IQ testing results much lower than I expected. For example, someone with a score of 55 who was navigating living on the streets and selling meth, not just delivering etc, but actually a drug dealer, and I suspect this individual's effort on testing was extremely low but the psychologist did not include any sort of validity testing in the IQ test.
I saw a patient with blatant BPD, but who herself wanted an autism diagnosis get said diagnosis from a psychologist after hours of testing. She did not meet diagnostic criteria for ASD, but certainly screamed extremely loudly when you tried to explain that to her.
Also a patient with BPD, anxiety, depression who was adamant she was "psychotic" because she felt like people at work were always judging her; well she sought psych testing on her own and they diagnosed her with psychotic depression based on her self-report of feeling judged in social settings.
I've also seen defense experts in competency evaluations seemingly ignore obvious personality disorders (typically antisocial) and diagnose bipolar disorder, when the person's symptoms were clearly better explained by their antisocial symptoms and they never had anything close to a manic episode (outside of meth use). And they completed and billed for several hours of psych testing to reach their conclusions.
All this to say that psych testing generates a seemingly quantifiable result, but that actually requires the qualitative interpretation by a psychologist. So on the surface some people may get the impression that psych testing provides "the truth" or is somehow the gold standard for diagnosis when the psychiatric clinical interview is unclear, but that's really not true.
Not to dog on all psychologists here. But the qualitative nature of the evaluations provides wiggle room to draw potentially questionable conclusions, like a person with ADHD (who just happens to be paying me cash) does in fact have ADHD, when maybe they don't. But wouldn't it be great to just keep getting super easy cash referrals for ADHD evals?