The effect of SSRIs on depression is moderate but statistically significant:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5667359/
I suspect the response varies greatly by individuals: some respond a lot and some don't respond at all.
My experience with SSRIs was very positive. I took a low dose for depression for a few years and it helped significantly. I went off it a few years ago and my depression hasn't returned.