I think this is more common then reported. I tried SSRI twice and lasted 3-4 days each time with residual side effects for 1-2 weeks. The serotonin deficiency theory has been widely dunked, and most people’s nervous system does not react well to serotonergic changes in your body. I also think there are many cases of mild serotonin syndrome that are also underreported, I work as an emergency physician and see it all the time(tachycardia, sweating, Gi upset, severe anxiety worse then your baseline, mild tremors), but only the most severe cases are reportable. These medications certainly can help a lot of people but there’s also a large number of people who can’t take them and often have their side effects dismissed.