I don't think that's quite accurate. We know first-order biochemical effects for a lot of drugs. There are always side effects, but they're side-effects, and those are largely understood as well.
Psych drugs have difficulties because they aren't used for their biochemical effects, they're used for their psychological effects, and psychological effects of biochemical processes are poorly understood.
Psych drugs have difficulties because they aren't used for their biochemical effects, they're used for their psychological effects, and psychological effects of biochemical processes are poorly understood.