You want the unsprung mass to be as low as possible. As in you want the wheels to be much lighter than the rest of the car for a smooth ride. The wheels should

You want the unsprung mass to be as low as possible. As in you want the wheels to be much lighter than the rest of the car for a smooth ride. The wheels should bounce around but the rest shouldn't. Newtons law and all. Putting engines in wheels causes a very rough ride. Even if the engine is light you're still likely creating an order of magnitude more unsprung mass. That's an order of magnitude rougher ride.