The issue isn't the cookers themselves, it is the fact that they require electricity, which is the real hazard. If the unit is in the rain, so is the cord that brings electricity to the unit, and no matter what, water and electricity do not mix. Making the cooker totally waterproof will not resolve this.
The second issue is that by having the cooker plugged in during a storm, it is grounded to the power supplies ground, which should be earth ground, and makes the cooker's large metal mass a lighten rod.