there are lots of differential diagnosis problems across mental healthcare
And there always will be, because people are complex and there are multiple ways to look at them; why is the medical model [edit: in this area, in psychiatry] so highly valued when it's just a series of failed and incomplete models propped up by drug company research? Medicine got its prestige by being able to diagnose definite problems with definite tests and offer definite treatments with definite results. Mental health medicine is largely absent those traits.
The landmark theory of depression, serotonin deficit, has been thoroughly disproven, and the drugs that theoretically addressed that deficit turn out to be very slightly better than an active placebo.
My conclusion is that society never stopped needing priests and witch doctors.