On both sides (and this is from a responders viewpoint)
1 Mandatory evacuation orders are meaningless. Only if you are in IMMINENT danger (like your home is on fire) can you be forced to leave your private property. However, if the disaster that caused the evac order is endangering you, the responders may not be available.
Around here, mandatory evac orders are issued most often for brush fires and hurricanes. Refuse to evacuate, get trapped by the fire, and responders may not be available to rescue you.
2 Leave your property in such a situation, and you are fair game for curfews and checkpoints designed to prevent people from looting the empty and damaged homes. Even people who are your neighbors can be looters.