There are three factors that destroy nutrients during cooking. Food is never heated hotter than 220 deg. Oxidation is practically eliminated and water is not a problem. 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.