As stated, you can loose your warranty. The purpose of the underlay is to help deaden sound when on the floor, and to take up any minor imperfections in the sub floor. The 2 in 1 underlay is also a vapor barrier in situations where it is required.
You can expect some flex in the floor until the underlay compresses. This will only happen with time and use. If the flex is in one area and not in others, you should check the sub floor for being flat. I can not say what the specs are on the floor you are installing because it is different for each manufacturer. Most allow only 3/16 to 1/4 inch of change in 10 ft.