I think given their abysmal track record and considering they only seem to work for half the people that take them, they aren't much better than a placebo.
Psychology isn't really a rigorous science like chemistry or biology. The biggest contribution that psychology has made was to separate people presenting various symptoms into diagnostic categories. It's pretty easy to say, this guy hears voices, this guy is depressed, this guy has post traumatic stress... but treating people, where it's possible, has been somewhat of a hit or miss for psychology.
I think the entire chemical imbalance hypothesis is a load of crap. There's no scientific test to diagnose anyone with any mental illness, it's just people labeling different behaviors. I think if someone is depressed, it's most often not because there is something wrong with them, or their brain, or the way they perceive reality, rather depression is a perfectly reasonable response for someone who is crappy material circumstances... poverty, tragedy trauma, etc... I would expect anyone who isn't a sociopath to experience depression and grief as a response to material circumstances that are less than ideal... but since it's much harder to change the world, or a persons circumstances, psychology often focuses on trying to change the person, or at least to try and teach them to be complacent in shitty circumstances. For a lot of people it might not be good enough to just live with depression, if nothing is ever done to address the source of their depression (soul crushing job) then that person will continue to be depressed. This is why medication I don't think helps most people.