Steaming - Food is heated to 212 deg, the temperature of steam. Oxidation is a big problem, vitamins and minerals are carried off with the steam. Water is also a problem, because water usually collects at the bottom of the steaming bowl. Pressure Cooking - Food is heated over 250 deg. Oxidation is not much of a problem because most of the steam is being pressurized and not escaping.
Water is a problem because water collects at the bottom.
Oxidation depends on the seal of the lid. It can be a big problem if no lid is on the pot. Water is a big problem. Unless, the water is consumed, as in soups, this method of food preparation is not recommended because water pulls most of the flavor and nutrients out of the food. This is an extremely high temperature, this is one of the worst methods of food preparation in terms of preserving nutrients.