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.
The conditions within the room's training area are extremely punishing on the body. 10 times Earth's gravity, temperatures that fluctuates rapidly between -40 and 50 degrees Celcius, and 1/4th the air pressure of Earth; try doing an intense workout while during arctic, desert, and high altitude conditions at the same time with extreme gravity added on top of it. Then you add the psychological toll of being in such an empty environment for extended periods.