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.
Can you find me any psychiatrist that's pushing the chemical imbalance theory? Every single doctor I know thinks chemical imbalance is bunk.