Iron core motors do suffer from an effect known as "cogging". Certain design changes to the motor can limit the effect, but these tend to lower the maximum torque. A motor at rest tends to settle at a point between magnetic poles. What cogging means to the servo designer is that it takes a certain amount of motor current for the motor to climb the hill, and thus for a purely analog servo amplifier, a bit of error is needed before enough currect is generated to offset both cogging and friction.

Coreless motors do not have the cogging effect, so less motor current is need to start movement.

However with digital amplifiers, motor type is less important.