No free lunch carries some weight in this context. If there is a chemical that has a very positive effect in humans, there is probably a reason that humans didn

No free lunch carries some weight in this context. If there is a chemical that has a very positive effect in humans, there is probably a reason that humans didn't evolve to produce that chemical or exist in the state that the chemical induces. That reason could be related to what you suggest, that the actual effect is a mixed bag and doesn't wind up worth it. It could be that our environment has changed to make it beneficial and evolution hasn't caught up. It could be that dreams are already the same thing and we're not sleeping enough or the right way, so psychadelics are filling the gap. In general, though, it's that the benefits are outweighed by costs. No free lunch is a reasonable prior in biology.