For one thing, the plastic used - HDPE - has a super low melting point. If stacking cages, the heat rising from the lower cage's reflector would be enough to cause structural problems in the floor of the cage above. If not stacking, you can get away with a bit more wattage.
To help with this problem, some people have gotten that aluminum roof flashing sold at Home Depot in a roll, and lined the inside of the recessed heating dome to protect the plastic, then run some under the floor of the upper cage.
I finally just installed a socket and utility box inside my Vision cage (model 322, no heat shroud). Got a nice wire bulb guard from Big Apple Herp. Heats the cage a little better with less wattage.