Regarding helping people with depression, the numbers don't really support a strong effect especially against an active placebo that makes you feel different like atropine.
[edit] Don't misunderstand, I'm not saying that there's not a role for pharmaceuticals. Just that the data for SSRIs specifically is not nearly as compelling as the quantity of prescriptions for it would have you believe, and they probably shouldn't be front-line treatment.
What we call "depression" seems to be a complex bundle of brain mess. We group by symptom but unfortunately need to treat by cause and they are not 1:1. So SSRIs are a tool "we" have on a population level - but each individual depressed person is their own situation.
Remember that "better than a placebo" is a population-level measure. Individual people will find a SSRI helpful or hindering as they go. I think you are mixing...policy ("we shouldn't assume this works for everyone") with drug efficacy ("SSRIs don't help more than a sugar pill"). How each persons' depression works is different and up-regulating serotonin is helpful to some subset of people. We don't know why and it's not the whole population, but talking about them in this way is not helpful in my mind.
Yah, serotonin is involved in a lot! I don't think that's a problem? You aren't saying it directly, but I feel like you are pointing to SSRI side effects as if they invalidate that SSRIs help depression. That's not true! People can choose if they want therapy or SSRIs or both. If your doctor has been telling you serotonin dis-regulation directly causes depression that's probably wrong - but if they tell you that SSRIs help many depressed people that's right.