Is there in explanation for the failure rate of DECs?
You are saying that the US training level is similar to other main players. Maybe yes, but a lot of DECs were trained only on narrow bodies, with their engagement profile, or even only on RJs.
I know these jockeys don't like to hear this, but it sure makes a difference how many tons you haul, for how far and into what environement.
There lies the biggest obstacle to overcome and the main criticism from FOs about working with such DECs.