Over the last several months we have had many customers call us trying to resolve an issue with their off-camera shoe cord. They have a tendency to break or wear out where the plastic shoe mounts to the bracket. The reason that the shoe mount rails are made of thin plastic. The way to reduce the risk of breaking off the off-camera shoe cord is to connect it via the 1/4-20 thread. The screw mount is much more sturdy.
See the two examples of Custom Brackets mounting plates below. The first shows the universal "FT" shoe mount and the flimsy plastic connection. The second example is the much more reliable "FCN" plate.