Since acrylic is not rigid, there will be flex in it somewhere, you just may or may not be able to notice it. This is aggravated by the fact that acrylic absorbs water. It is most noticeable when one side is wet, and the other is dry, but uneven absorbtion will alter the "straightness" of a chunk of acrylic. This alone, aside from the lack of structural strength when using silicone to attach it to glass, makes glass the more suitable material to use, when water is involved.