So, they sucked at being regulars. That was what I said. I believe I was specific to the American REGULARS. Denying that the US had fine irregulars is like denying that the British had a navy.
However, operating in a capacity as regular soldiers, much like the Vietnamese you mentioned, they were rather poor. Perhaps 'sucked' is an over exaggeration, but this is a board about a game, not a semantics class.
So, to be perfectly clear:
American regulars were poor quality.
Once they ceased to be regulars, and began operating in the capacity of IRREGULAR soldiers, their quality improved greatly.