Short circuiting ('frying"). You can't run power through tinier and tinier spaces, and expect everything will last forever, for every unit delivered. In the case of electric power, that's the insulation blowing in some coil. How often/how long until it happens? Depends on quality controls in the plant, and safety margins on the drawing board/screen. In the case of the Prius, there's also the engine computer trying to prevent certain harsh operating conditions.
There are no brushes, so those won't wear out. But for a major application like this, a motor with brushes would have some means of disassembly and replacement.
Carlos