Scaleable speed stabilization (back EMF) compensates for load by providing more voltage to the motor. If there is a mechanical problem with the locomotive that greatly increases this load you can prevent damage to motor by setting CV57 to limit how much throttle can be added due to BEMF.
Digitrax Scalable BEMF uses CV57 to control how much compensation authority the BEMF control loop has. The higher CV57 is (0-15 range) the more back-emf the control loop can crank in; a value of 0 effectively turns off BEMF compensation. The value in CV57 takes into account the different throttle levels as well. A low value of CV57 limits how much throttle can be added due to BEMF feedback/compensation, so this effectively limits the stall/short circuit current that can be applied to the motor that is primarily set by the throttle setting.
I do not know of a smooth "linear voltage" output BEMF/PWM drive. All decoders employ saturated-switch DC motor H-bridge drives- they are not capable of smooth DC due to power dissipation limits. CV57 does limit the effect of BEMF in our decoders.
Excessive track voltage may cause added heating in DC motors at low throttle settings, but on the whole they integrate the input energy into their output and iron core units are reasonably robust. Ironless motors are very delicate.