The knowledge existing on barotrauma in divers medicin is applicable for those skinsuits. Any larger pressure difference between different parts of the body may cause small or larger injuries. Those injuries caused by pressure differences are called barotrauma.
The air pressure at the mouth should be equal to the pressure on the skin for every part of the body. Pressure differences may impair blood circulation, lymph flow and air flow into and out of the lungs. The minimum blood pressure is at least 80 mbar, any pressure difference should be much smaller. The difference between chest surface and the air at the mouth should be smaller than about 40 mbar, larger differences would be exhausting and may impair the blood flow between both chambers of the heart and the lungs. To high pressure differences may cause severe and dangerous damages to the lungs tissue and to the heart muscle. The muscles of the chest used for breathing are not very strong, if they are exhausted by larger pressure differences, a dangerous situation may occur when less than necessary oxygen gets into the circulatory system and not enough carbon dioxide is removed from the body.
But this restrictions are also a problem when such a skinsuit is already put on but the helmet not yet, the pressure on the chest would inhibit breathing in. How would you don or doff a skinsuit and the helmet without generating to high pressure differences between chest and mouth during the procedure?
The small pressure differences should be sustained for any phase between full inhalation and full exhalation for any surface part of the torso. For the arms and legs, larger pressure differences are possible than for the torso as longs as blood circulation and lymph distribution is not affected.
But what about cooling of the body? If the sweat could not evaporate and thus cool the body, the body would overheat and the performance of the wearer is substantially reduced, higher temperatures will be dangerous. Either a liquid cooling garment should be worn under the suit or the suit should be permeable for water vapour. But the skin might be impaired if to much water is removed from it. If too much oxygen diffuses from the tissue under the surface of the skin and out of the suit, damage to the tissue may result when the suit is worn for many hours.
To avoid skin-abrasions near the joints of the body caused by the moving parts of such a skinsuit would be very difficult if the suit is worn for many hours and on many days in sequence.