Im not surprised. Being the majority of the electric fleet, M7's are run to the ground, and doing 70+ mph and braking from those speeds constantly would definitely do a number on the wheels.
To be fair, SI's express network is trying to do for the island what the subways do for the other boroughs. I can't speak for UP, as it should be a pretty easy operation with only 4 routes, but SI runs their buses to the ground considering the volume of service during the week.