My Hammer plastic spare ball with the Carbon Fiber cover ended up cracking around the thumb hole, just as all my other plastic balls eventually did. The Hammer warranty does not cover this.
I finally got fed up and bought a Storm Mix. It's highly polished urethane, and reacts almost like plastic on most conditions. I've yet to throw on a condition where I couldn't successfully use it for throwing straight at spares. After several hundred games, there are no signs of cracking. The bottom line is that uretane is much tougher than plastic, especially if you clip or roll over holes.