At one time, I was employed to do some tests on sundry connectors that were intended to be used with cords. One of the test was to hang a weight from the cord, to see if the connector did actually prevent the strain from being transmitted to where the wires were connected.
I did test several such connectors on Romex, rather than 'extension cord' material. In every instance, the connector failed to hold on to the romex well enough. That is, the outer jacket would work free of the connector, and you'd be left with the wires holding the weight.