On the wheel, it's unsprung mass. That extra mass hurts the ability to the wheel & suspension to quickly respond to bumps, and causes energy to be lost into the suspension and tire.

It's even a problem on trains - early electric trains had connecting rods between the motors and wheels just like steam trains. Later ones have the motors in the bogeys, but the motors are sprung and can swivel on the same axis as the drive gear.

Since nobody managed to ELI5 this one:

The tire wants to go in a straight line (inertia).

When a tire bounces over a bump, the springs push it back down onto the road. The heavier the wheel and the arms, the longer the tire has poor (or no!) contact with the road. No contact = no handling.

It's like hydroplaning on a dry road. Scary shit. Potentially fatal on a curve.

Other people answered the hanlding question but there are also durability problems because of the additional shock loading introduced by the extra weight. You wouldn't drive a car without suspension at highway speed. It would kill tires or other components in short order.

Now replace the entire car with a 75lb electric motor in each corner and you've got the same problem but less extreme. It beats the crap out of the rim and tire. This wouldn't be too big of a deal but the hub motor all but forces you to use big rims and low profile tires which are already not that durable for reasons you can Google.

the wheel must change direction and move quickly to follow imperfections in the road surface. Increasing this weight decreases the wheel's ability to follow the undulations. Increased mass also contributes to centripetal force, inhibiting the wheel's ability to change yaw (turn). It acts like a gyroscope.