The gears which drive the folding of the mirrors are prone to failure, as they are plastic/nylon and fairly brittle. And then the motors themselves can fail as well. You can tell the gear is toast on a 'dead' mirror by pressing the folding button and then listening for the motor sound----sound but no movement = bad gear.
My driver side mirror has no power-folding capability remaining at all. Passenger side mirror power-folds inward but won't fold back outward. Not sure I care enough about this particular feature to buy new mirrors, and I'm not going to drive myself crazy tearing into the mirror bodies to replace the gears. Regardless, I still have the angle-down-during-reverse feature when activated.