Is there any limitation in how quickly/often I can change a 6V DC motor direction of rotation?
&quot;Slamming&quot; the motor from one direction to another will produce both electrical and mechanical stress, especially if there is any load connected.  It would be particularly horrible with a gearbox.
Since you have an MCU generating PWM what you should do to make things more gentle is to profile the movement: gradually ramp up the speed, then ramp it down and ramp up in the other direction.
Even a rather short ramp will be drastically different from the approximation of &quot;dividing by zero&quot; which occurs if you give a direction change with no transition time to divide by and force the limitations of the parts to create their own.