If you use 1 motor per wheel, at full torque you don't have double the power but the same power as one motor on the axle. This is the fundamental disadvantage of using one motor for each wheel, which is probably why Tesla engineers didn't pursue this route.

In addition, the extra weight also adds up, no matter how tiny the motor is. Essentially you are making each wheel heavier, adding to suspension woes.

The greatest advantage of individual motors is in cornering when you can precisely control how much power each wheel gets, and obviously on icy surfaces when you can lower the power on the slipping wheel or disengage it completely and drive power from even one wheel which still has traction and grip. Nearly the same result can be obtained by torque vectoring, which is what Tesla employs. So for may be 20% less accuracy you can do away with the individual motors.