Every central vacuum power unit -- actually, every product -- has good points and bad points about its design. Trade-offs are unavoidable in life. Sometimes though, a simple and easily corrected design flaw can become a hazard.
Such is the case with certain central vacuum power units, having flat tops with unprotected motor intake vents. This issue occurs in certain models of brands such as:
With this design, there is nothing stopping an object placed on top of the central vacuum unit from restricting the motor cooling airflow, leading to motor failure. Power units that use disposable bags are especially susceptible to this problem, since atop the unit seems such a sensible place to store extra bags!
Here's an example of a unit as I found it:
This machine is an Astro-Vac, which uses disposable plastic can liners. The owner's manual and spare can liners had been blocking the motor intake vents long enough that the heat from the motor had melted the plastic and started to scorch the paper. The owner was amazed to see how lucky she was that this was found in time, and how close she came to needing a new central vacuum unit (or worse!)
Units like this are not inherently dangerous, as long as you, the owner, are careful not to obstruct the cooling air vents by placing anything on top of them. I have even taken a marker (or you could get fancy and use a label maker) and written a warning in big letters.
If your machine has only a lid on top, and its motors are mounted on the bottom (like Filtex, MD, SilentMaster, Air-Flo, etc.) then it is perfectly okay to continue storing bags on top of the machine -- in fact, that's my favorite spot for them. When in doubt, however, the safest rule of thumb is not to place anything on top of your vacuum, and keep objects clear from the sides, too -- this promotes better cooling, and longer motor life.