Cedar shakes are prone to Cupping, Splitting, Warping, Mildew, Moss and Algae growth.  A wet cedar roof is often times spongy.  The water soaks the wood all the way through when it rains.  Accordingly, it is necessary to treat wood products frequently and keep debris from accumulating on your roof.  This makes maintenance costs of cedar roofs much higher than that of composition.  And when the water dries with warm tempuratures, the risk of fire is increased.  Poor fire ratings are enough to make many people want to swith to something else.  After all, insurance can't bring back or pay for all of the intangible items in your house.