I don't know anything about pot, but it's pretty evident that conducting controlled studies hasn't been terribly easy, so maybe as that becomes more of a possibility, we'll learn something.
In this case it's mainly attributable to the DEA's scheduling system. People have to rely on personal experience because there isn't proper research being done.
This is my greatest frustration around the use of drugs. I think it's ludicrous that more research isn't done on substances that millions of people take.