When humidifying in the gas to gas mode, no additional energy is required as the moisture transfer occurs as a first order chemical reaction, driven by the differential in the water vapor pressure. In the Water to Gas mode, liquid water is transported from one side of the membrane to the other, and the water droplets that are transported through the membrane and then appear on the other surface are vaporized into the dry gas stream. This vaporization is naturally endothermic, and unless the water is heated and circulated to a constant temperature, the operating temperature of the membrane will continuously decline, lowering the performance of the humidifier.
One point to make is that if you are not heating the water enough, or the circulation of the water is not sufficient, it is possible to get localized “cold spots” in the humidifier, which could lead to reduced performance, or even liquid water droplets exiting at the outlet (which might be misinterpreted as leakage).
This also means that any component in the gas (or aerosol stream) being humidified would be affected by the liquid water droplets that will be present on the membrane surface. If those components are water soluble, they might go into solution with the water droplets.