I thought the Soma in BNW was more like a type of depressant, like an opioid.

Psychededics are unpredictable. We know the government looked into their use as thought- and behavior-manipulating tools (MKUltra) but the results proved too chaotic. You might get a manchurian candidate...or the unabomber. But most likely just a renewed outlook on life.

Psyches basically raise the "temperature" (in machine learning parlance) of the brain, increasing crosstalk. This can jostle folks out of a mental rut. But it can also create positive feedback loops of upheaval.

Soma would only work as it did in BNW if society controlled essentially all sources of information - which is essentially the whole premise.

hallucinogens where taken by hippies, it was probably a way to put a few extra commies and lefties in prison