CONS:
A stack can be unstable. Don’t laugh, it COULD be tipped over. Do you have any kids or pets? What if a largish snake escapes and climbs up your stack of tubs?
Not as secure. Some animal sometime is going to probe the right spot on a lid hard enough and escape, especially from the topmost tub in your stack. Tiny snakes escape with greater ease than larger ones.
Works against timely maintenance. It is easier to slide out a box in a rack for a quick check on the cleanliness of the container then it is to unstack a stack of boxes to check on the one on the bottom.
More difficult to fine tune the heat. How ARE you going to provide supplemental heat to a moveable stack?
Add enough weight and a lid may fail. Either heavy occupants in the tubs, or heavy weights on the top lid to contain all the occupants in the stack, will probably produce more forces than the lids were designed to withstand. A lid on one of the lower tubs may eventually crack and perhaps cave in due to the weight from above.
PROS:
Other than being spared the expense of the rack, I can’t think of any.
PLEASE NOTE: As much as I hate to admit it, all examples are from personal experience.
-Joan