Depends on the set-up - full-on braking with a large motor spinning fast and lots of (moment-of-)inertia could rip the motor
off its stand! With a small hobby motor I don't think there will be any issues...
With electric vehicles the electromagnetic braking is under PWM control so the braking force can be controlled
to the desired level, avoiding wheel-lock or damage. Full-on braking is equivalent to stall-conditions so that the
current (and thus torque) is limited ultimately by the motor's internal resistance.
DC motors produce torque pretty much directly proportional to current, and back EMF is directly related to rotation
speed. When under normal load the applied average voltage is roughly the same as back EMF, only the difference
between applied voltage and back EMF is available to drive current through the windings. Braking is simply applying
0V to the windings (thus full back-EMF is available to drive current through windings)