Oh count rare ...
Tranzorbs are superior to MOV's in just about every way except cost. An AC tranzorb is two high voltage zener diodes in series back-to-back. They have a way faster response time. AND (the biggie), they do not degrade nearly as much with each transient hit. In round numbers, an MOV's tranisent performance decreases about 10% with each big hit. The clamping voltage increases, the response time increases, the clamped impedance increases, and the "knee" between high and low impedance gets softer. 4 strikes. At work (small and medium-sized MIL electronic systems, air, land, sea, AC, DC, whatever) and at home, I use nothing but.
Cons: stiffer leads, more careful handling when bending the leads to prevent over-stressing the internal diode junctions, cost.
ak