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.
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.