The preponderance of evidence (from Hubbard and others) is that he was mentally ill before, during, and after Scn. From many sources, you can learn of crimes of Hubbard and behavior inconsistent with what we could consider enlightened or great by any standard.
That doesn't mean that everything he posited was false or wrong. He pirated a number of good ideas and innovated others. His biggest wrongdoing in my opinion was to make Scn a "church." Since religion is a matter of faith, doing this removed Scn from any requirements of science and proof. If the tech does work, its workability could have been proven, and it would have gradually gained more and more support and currency. But as a religion, it becomes a major yawn, a matter of faith, and is denigrated into a debate of "it works!" and nuh-ah!"
Overall, I think it is much more likely that Hubbard was just a human with a mix of greatness and madness than someone who saw to the bare truth of the universe. After all, if OT VIII were real (full cause over MEST) then Hubbard could cinch the argument once and for all by simultaneously appearning for a day before TV cameras at every mission and org on Earth. That would prove Scn works and the clearing of Earth could proceed apace. How _unethical_ not to do that since it would be the greatest good for the greatest number and for Scn itself.