It is also pretty hard on the transport system, particularly if you are using heavier weight magnet sheet. The weight, thickness, smoothness, and hardness of the back of the sheet can conspire to actually grind down the friction surface of the grit rollers. I know the system can pull pretty heavy rolls of banner or vinyl, but the physics is a little different since those materials are pulled off the roll, whereas with a magnet you are moving the whole weight of the sheet around, and when cutting it you are repeatedly moving it back and forth.
Once in a blue moon it is probably okay, but if you are building a business around magnetic signs and need to cut them daily, or even weekly, we don't recommend it.