One problem is that there are so many varietals, and so many different vehicles for administration (smoking, vaping, eating, tinctures, lotions, etc) — that make it hard to announce, globally, that "marijuana does X to your brain."

Really, the most we can say after a given experience is something like "Strain Y, when inhaled as a combustible, appears to show effect X."

Another problem is the weed that's available for experiments in the US. There is one — and only one — weed crop that the FDA will approve for clinical studies. From a farm at the University of Mississippi. (https://www.latimes.com/nation/la-na-pot-monopoly-20140529-s...)

And according to a weed researcher I spoke to (Sue Sisley - https://medicalcannabis.com/about/faculty/suzanne-sisley/), that Mississippi weed is very low quality — aged, with a lot of stems, seeds, and adulterants.

None of this is to say that weed can't be beneficial.

But I'd argue that anyone making any global claims about weed's health benefits are, at the very least, overgeneralizing.