Mostly that two full drivetrains are expensive, heavy, and labor intensive to install. You end up with the worst of both worlds. BTW, the volt was advertised/

Mostly that two full drivetrains are expensive, heavy, and labor intensive to install. You end up with the worst of both worlds. BTW, the volt was advertised/promoted as a serial hybrid, which was a really interesting idea ... right up till it shipped and it wasn't.