The main problem is as already stated the cw and pinion are no longer genuine parts. When Ford made them the tolerances and finishing procedures were much better, but they were made in far larger quantities and Ford could reject the ones that were not right.
I also think that the rebuilders of these diffs do not spend the time needed to build them as Ford intended. It is quite a lengthy job setting the depth of mesh and can not be done just fitting the shims that have been fitted previously.
A few diffs do fail, lack of oil and faulty or old bolts seem to be the main cause.