I got the definite impression that Heinlein liked Hubbard a lot when he was young, pre-WWII, but Hubbard's behavior towards women and in other respects had led him to quit trusting the Scientology founder by 1948. I'm not surprised. I wouldn't expose a child or vulnerable adult to the founder of Scientology. :/ At the same time, I suspect that Hubbard's bio is as you say very far from boring.
I'll check out Going Clear.