Its a problem because the UP outputs cannot get the charge out of the
motor driver fets fast enough. The result is driver fets spending too much
time in linear region, overheating. Possibly not even getting fets into
saturation. This is all dependent on PWM frequency, thermals, etc..
A good motor driver H bridge has thermal management, gate charge
management, current limiting...Wise to use one.
Infineon, On Semi, TI, ST......
Regards, Dana.