Biggest issue I've seen is the ability to fold a hem closed is relying on the magnetic force, and sometimes doesn't do as well as the apron brake would. If bending aluminum the magnet has no effect on the material so the capacity seems to decrease significantly.
Be sure to visit this page and click on #7, Getting more out of your Magnabend. Some good helpful hints.
http://aaybee.com.au/Magnabend/Magnabend_Homepage.html