For alloy forks, it's metal fatigue. More expensive forks are made of magnesium alloy, which has an even lower fatigue resistance than aluminium. The drop-outs are designed to take an oscillating force in the up and down direction, not the substancial torsion force.
The main way I've seen the drop-outs break is when the fork has lawyers lips and the installer didn't file the drop-outs deeper t re-centralise the axle, so that the nut or washer tightened on the lip and popped half of the drop-out off. Others I've seen happened randomly after some time, which was probably metal fatigue, but could be because the nut came loose or wasn't tightened enough. The drop-out itself is not strong enough to resist the motor's torque. Any will break if the nut isn't tightened.
If everything is done right, it can work, but most people don't know about torque arms, centralising the axle, nor do they know what their forks are made off. Then, there's always the possibility that the nut wasn't tightened or came loose. Whatever it is, failure is catastrophic and is likely to lead to injury when you go straight over the handlebars to land on your head. There are so many ifs and buts with such serious consequences that it's best to say just avoid it unless there's no choice.
