Polyurethane bushings will be made with more resin and less filler as they have to flex somewhat, think sway bar end links, bushings and the like. The poly spacers I've seen are made with more filler and thus are firmer because they're being designed to hold up the weight of the truck. That extra filler makes the poly more brittle and thus more apt to cracking. I'd especially question ebay poly spacers because you know they're being made in China for 6 cents and they are using as much recycled content as they can, which makes it harder for them to control the formulation of the polyurethane.
Sorry, just realized that was quite a stream of thought.