This isn't a secret. I was required to be CPR Certified for a decade. Every class I ever took, the instructor made it clear that it only makes a small difference in survival outcomes, in order to teach students that "you shouldn't be scared to do it." If done properly you should break ribs. They also teach this in classes. I don't think anyone is under the illusion that CPR is a hugely high-percentage life saving technique, except maybe the public at large, but the public at large believes a lot of stupid, untrue things.