Scientologists claim to know many things. Like how intergalactic 707s brought the frozen souls of millions to earth where they thawed in volcanoes and started inhabiting primitive humans. Which tells me that much of what they claim to know is BS.

Lets face facts here, Hubbard was a failed science fiction writer, his early writing are shockingly bad, in part because they outline the basics of what would become Scientology, who understood that people want to belong and are generally willing to swallow anything if they can feel like they are part of something larger than themselves. Hubbard combined people's ignorance and awe of science with the self-improvement movement and abstract mysticism. Then structured it to make a profit from it.

The height of the bomb shelter movement was in the late 50s and hit fever pitch in the Cuban missile crisis.

Which isn't to say that some sort of shelter might not be a good investment. Even a simple but well engineered shelter could be useful in extreme weather, wildfire, or other calamity. What the Scientologists, or any other group, do shouldn't shift your estimates of what is best in your particular case.