The existence of undiagnosed disorders and the potential presence of effective coping skills/supportive stable home environment/unstressed personality/late-onset symptoms would preclude reliance on prior indicators of determining fitness for a given psychedelic.

Family histories are not always clearly diagnosed. That hypothetical uncle you've never met and no one talks about? Unknown mental health flag.

Self-diagnosis of a mental/emotional state that calls for drug-taking is itself a clue that not all is so normal in the individual's mind--or is it something wrong in the world the person wants to relate to better? Who can know but the one supposedly experiencing an unnaccepted state? Observing/deciding/dosing by the selfsame suffering/diagnosing/tripping individual sounds risky, because if the downside is a long future of some degree of insanity, it is only a variety of Russian Roulette.

To allow scientific studies, fund them, possibly create therapies is to also conquer the stigmas involved in substantial ways and that process is tectonically, multi-generationally slow (or impossible).