A friend of mine has a voltbike fat bike with a rear motor, and he wishes he went for a mid drive. He has burned out the rear motor twice. He is a strong rider, and rides hilly trails. The vulnerability of the rear drive is that it can't get an assist from a lower gear. If a heavier rider pushes on the uphills, the motor can be overloaded.
Anyway mate, if you have hilly trails in mind rather than road commute type use, you might want to consider a mid drive. The rear suspension is not really needed on a fat bike, the fork and softer tires make the ride decent even on trails.