 It's very simple to make a quality polyester recipe, but much harder to make a top quality polyurethane.If the polyurethane is too soft it will split apart when the hold is tightened, or the bolt might get pulled through the hold, or the hold will flex on the wall or could polish (become slick) quickly.If the polyurethane is too hard it will be brittle (like polyester) and the edges could chip or it could crack when tightened (also like polyester).