I've always had a problem with these and Bill points to a valid concern about expansion causing cracks, but the stress that concerns me is from the temperature gradient from the outside surface to the inside surface of the concrete structure.
This is the primary mode of natural failure of most crystaline ware and it should also apply here. As the outside surface is warmed it wants to expand but the inside surface, being cooler resists that expansion. This places the outer surface in compression and the inner surface into tension. This is not a bad thing as it toughens the outer shell, but when the sun goes down the outside will be under tension which means it's lookin' for a flaw, stress riser, or any other excuse to crack. Given the thickness of the typical wall these forces could become quite large.
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