Diagnosis is still left to the subjective assessment of the practitioner before prescribing powerful psychotropics that DO change brain chemistry in unpredictable ways, and cause a host of side effects often worse than the original symptoms.
So the argument today that psychiatry lacks any scientific basis and rigor for its practices is still very valid and legitimate.
If I do a study showing that handwashing in surgeons measurably reduces risk of infection, does the fact that I don't know about microbes make lacking in scientific rigor and remove its scientific basis?