These drugs seem to be karmic accelerators, and they seem to intensify time by factors of 100 to 1000+, which means you can get months or years worth of experience in a few hours. If you're on a path "upward" (ignore the subjectivity of spirituality and growth for now) the drugs will push you along, you'll learn a lot, and have a beautiful experience. If you're headed for, or at risk of, crisis or a disaster, they can bring it forward with a lot of force... and it might be better, for many people, to have more time to deal with such a thing.

Monitored use with a skilled therapist is probably fine for most people (and I wouldn't be surprised if psychedelics were much more safe than what's currently being used for high-grade mental illnesses, because many of those drugs-- legal and regularly deployed in mental institutions-- are nasty) but the recreational/hedonistic use pattern you see in our society (which, for the most part, is materialistic, thrill-obsessed, short-tempered and crass... and using LSD or psilocybin doesn't change that aspect of a person unless he has the right intentions) seems to have minimal expectancy and a lot of variance.