Since the beginning of the era of modern psychedelic research, no one has developed any serious longterm mental illness as the result of taking psychedelics in a research setting. And this is out of thousands of people.

I think part of the reason is that it's actually a lot easier to determine if someone is predisposed to mental illness than it actually is to determine if the person sitting in front of you has a mental illness. E.g. if one of your parents was bipolar or you had a relative who committed suicide then you'd be immediately excluded from any trial. And because they are only doing this research on adults, this means that participants would have all had 20+ years to figure out if their parents have signs of mental illness or whatever.