The problem with trying to maintain a level attitude in a rapidly rotating helicopter for more than a very brief period is (as SND wrote from personal experience) that cyclic inputs may not have the expected effect, even if the pilot was able to overcome the visual confusion caused by the rotation. Think about the trimmed position of the cyclic. The normal reference point for the fixed main rotor swashplate constantly changes once rotation begins. After 90 degrees of fuselage rotation, what was a pitch is now a roll, and vice versa. After 180 degrees of rotation, what was a pitch up is now a pitch down.