Same for antidepressants - often the same drugs (especially SSRIs) are both antidepressants and anxiolytics, but both labels are useful descriptions of the meds' common effects. Antidepressants in particular happen to be fantastically idiosyncratic drugs, but on average they do what they say on the label.
Part of the problem is that, due to the ill-understood nature of the brain, good psychiatrists need to know strange drug interactions and ill-studied minority reactions in order to serve the full range of their patients. This edges on art rather than science, which is always a recipe for cognitive biases to gain control.