Never had one break. But on mine the part that lifts up is, I think, metal.

I have many of the older close couplers. Occasionally, thry get hung up in a derailment, but usually are fine. I've had one or two of the metl parts crack off because they were bent and broke from over straightening upon reassembly. Use a nedle nose plier to squeeze them from the sides onto the plastic body after re-atatching. The metal part should pivot freely or its not aligned or too tight on the shank body. The current conducting ones are set up the same.

I assume we are talking about short couplers and I also assume that small bar is part of the Relex mechanism.
This bar just prevents the coupler loop from falling down over the hook, so it only must carry the weight of the coupler loop. Not much strength required.

In my experience the failure rate with those short couplers is very low. The small tips that extend towards the other coupler are at the biggest risk and will sometimes bend downwards or to the side.