TCM, as I recall it has something to do with the inherent toxicity of dopamine when it auto-oxidizes after being taken up by SERT into cystistolic/vesicular stores, as in, in the presence of surroundings which aren't base enough to handle it without being damaged. I bet this happens with methamp and even amp given the structural similarities of the three major monoamines but to a far lesser extent. I read this awhile ago though.
If down-reg of 5ht2a caused neurotoxicity, mushrooms/LSD, basically the psychedelics, and atypical anti-psychotics would be under even more scrutiny.
One could make the case that SSRIs are toxic. For instance, inflammation causes the bdnf to be released which leads to most of their neuro-protectiv/generative properties.
Nevertheless, in basic psychopharmacology, SSRIs have one simple action on serotonin, whereas meth/MDMA trick the body and insinuate themselves as endogenous.
THC has been shown to help reduce MDMA toxicity and fluoxetine to help recover the brain from mdma-induced damage, doesn't make it safe, or true for that matter. And despite similarities meth is quite different from MDMA IMO.
I'd be more worried about dopaminergic toxicity.