Psilocybin is a partial 5-HT2B receptor agonist. 5-HT2B receptor agonism causes deposits of collagen in heart valves (which will lead to valve disease). This is true for LSD, MDMA, and psilocybin.
A note is that psilocybin is only a partial agonist, and the dosages taken for micro-dosing may not be comparable to something like fenfluramine (a drug which does cause heart valve dysfunction for certain). There was a drug nerds thread lying around on Reddit where the theoretical receptor activations were compared. IIRC, psilocybin is not a major risk, as long as you're not tripping every day for months on end.
The risk comes from taking the 2b agonist regularly, that's why it causes fibrosis. Not only is taking the larger dose safer as long as there are no preexisting cardiovascular issues but microdosing has been shown to be no better than placebo
Microdoses also don't inspire hippocampal neurogenesis whereas larger doses do
Microdosing is not only not beneficial but it may be actively harmful
"...it is possible that chronic microdosing may carry a risk of fibrosis and VHD, which should be assessed in future studies. There is converging evidence that simulation of the 5-HT2BR over several months may lead to the development of fibrosis. Duration of intake plays a major role in drug-induced VHD, even if the substance is not taken daily (Connolly et al., 1997; Schade et al., 2007)."
