There's photos of these things being the only structure standing after an EF5 in Moore, OK:
https://oklahoman.com/article/384063...-moore-tornado



Since they are above ground and secured to the foundation itself they are less susceptible to rising ground water, which is a concern for underground (they can "float"):
https://www.news9.com/story/5e34c6c4...r-severe-storm


Some more examples of the underground problems: https://buildblock.com/buildblock-ic...nd-safe-rooms/

All in all, the consensus is that, if properly installed, they are just as safe, if not safer, than underground and easier to deal with/maintain.