I would fear any 'extra' stress on the mounts regardless of the material being plastic or metal.
Metal could just as easily crack rather than dent when abused.
There is the old myth that a 'mostly' metal lens or body is superior because it is a heavier weight.
The example is the older non USM 100mm Canon macro, and it is built like a tank.
Many users look at the 100L, pick it up and immediately dry inferior materials.
The point is this claim is made without researching either materials used to make those lenses.
If i drop the old lens the metal dents, and it can't be used.
If I drop the new lens the plastic breaks apart, and it can't used.
As other have stated the reliability comes from the 'true quality' of the material used to make that TC
Hold one hand on the camera body and one hand under the lens as often as possible.
I have used my Life Size Converter often on my 100L, always supporting both ends, and it has metal mounts.
The better you treat your gear the longer it will last. Many primes lens still function at 100% decades later.