In fact I'd go a step further and say that it's not only difficult but impossible to assess since it's pretty easy for the test subject to discern whether he was given a placebo or not, especially considering that the benefits are suspected to come from the induced experience itself and not the drug as a substance in isolation.

You can't control if exercising is placebo either, can you? But the consensus is that it's physically and mentally beneficial. I don't think controlling for placebo is of great use here although it obviously has to be taken into account.