I think your definition of "understood" is too narrow and perhaps that is your challenge. People have been taking many of these substances (e.g. psilocybin, THC, DMT, etc.) for thousands of years. Their qualitative, long-term effects are extremely well understood by the cultures and peoples that use them. My assumption is that you are WEIRD (forgive me if I am wrong), and the tendency we have is to disregard any data that wasn't created in a Western lab.