I've been investigating getting an enamelled cast iron dutch oven and am surprised to find that most of them don't recommend going over 400-450°F (200-230°C) (most even on just the base, so the lid handle doesn't appear to be the only factor there).    Given that normal cast iron can go much higher and I wouldn't have expected that heat limitation on the enamel...what is it about the pots that causes that limitation?  What risks are there in going above it? 
I think its because of the difference of the thermal expansion between the two materials.    Since the ceramic has very low thermal expansion coefficient and cast iron seems to have a rather high coefficient comparatively, then the metals will start to pull apart too much at higher temperatures.  The metal expands more than the ceramic under equal heat, so they start to separate and crack.  Apparently, at just above the recommended point, it may start to cause the surface cracks which are common (thanks @TFD). 