What is so dangerous about the Hyperbolic Time Chamber? When Goku, Gohan, Vegeta and Trunks prepare to train to face Cell Goku warns them that the Hyperbolic Time Chamber is dangerous enough that they can't train alone in it.

But there's no life in there. just a building and open empty white space. how exacting is it too dangerous to train in there alone?
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.