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.
It's been said that psychiatry is the new religion and psychiatrists the new priests for a reason. Any good actor with enough information can pretend he has a mental illness and fool psychiatrists. Not only that, you can have psychiatrists come up with different diagnoses for the same individual - even to the point of contradicting each other. And there is no scientific way to show one psychiatrist is correct or not.
Just like religion was the best we could do with level of knowledge we had in the past. Psychiatry is sadly the best we can do with the current level of knowledge. It's pretty much voodoo, but it's better than the voodoo that came before it. If neuroscience ( an actual science ) lives up to its potential, then we'll finally be able to toss psychology into the same waste bin we put astrology in and perhaps psychiatry can have a scientific foundation.