Total speculation on my part, but I assume that the hub motor being a mass added to the unsprung part of the drive train is subject to a lot of shocks and vibrations.
At lower speeds, these will not be so bad, but at highway speed, or even over 45 MPH, the force will be enough to cause issues, either in the metal that makes up the hub motor, or the epoxy (or what have you) that keep the windings and/or magnets in place.
I did some quick googling, but the scientific papers I came across I was not sure if they even applied, so I didn't link to them.
Protean, which makes in-wheel hub motors, says if made correctly, it is not a problem... but most of their current products are in urban, lower speed vehicles, like buses.
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Yet another concern that has been voiced about in-wheel motors is their supposed lack of durability. Placing the motors in the wheels, as opposed to under the hood, means they will get battered when the car travels over rough roads. They’ll also be pelted by water, sand, gravel, and all the other *detritus we drive over on a regular basis. Yes, these conditions could pose a problem, but we’ve shown through extensive real-world testing that it’s perfectly possible to engineer a product that can hold up to the beating.
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https://spectrum.ieee.org/transporta...more-efficient