You can use the exact same dose a month apart and will have wildly different reactions to it (both during the experience and afterwards (add digestive system weirdness to the list)); Nevermind trying to predict outcomes between different people, even if the dose is the same. And since the experience and some of the effects are based on your current mental landscape, it tends to bring more of what you are to the front (on top of your physical surroundings, sounds, smells). This can good or bad, and sometimes both. Point being that you can permanently alter your mind and body in unpredictable ways.
I greatly value my experiences with them but I don't touch them any more because I'm truly afraid of what they are capable of. I don't think I've been damaged by it in the long term. My own experience of course don't invalidate other's experiences or that it has helped people. I just feel that it dangerous in the same way that jamming wet fingers into a broken plugged-in toaster is dangerous. You might get lucky a few times but at some point the toaster will teach you it's ways, and you may never get to enjoy toast again. Well you might, but it might come in through a tube while you are drooling at clay in a mental hospital.
One shaman told me to never seek these things out. Wait for it to call you. Learn your lesson and move on. He also said not to push others into it. This is YOUR journey, their time will come sooner or later (meaning different lives, if you are into that). He also said not to try and force the substances/plants/medicines to give you specific knowledge/teachings/truths - they will show you what you need to see.
Sorry if I seem conflicted - I both want everyone to do it (hoping it would make the world better via empathy) and nobody to do it (because even a single person suffering a bad trip is not worth the cost (it really is hell x1000000)).
Hope it helps the discussion.