What is so dangerous about the Hyperbolic Time Chamber?
My answer is only opinion based but i think it has to do with the fact that nothing is there besides white emptiness. The "danger" comes from the psychological nature, as a solo person would eventually go crazy without any object reference. Everywhere around you is just an endless whiteness so that most likely takes a huge effect to your mental state.

Anyway that is the only reason that comes to my mind.