The show runners are supposedly professionals and this their only job. They said that they chose Arya to kill the NK because no one would see that coming. Like, I can't even...
Well, I think it is pretty likely that one could look around and find a nearly endless number of people who just aren't very good at their jobs...
But really, the issue is more that these guys are "jobbers" and what we really wanted, for the most part, was some sort of
art. It's sort of like we went to a super-upscale restaurant, looking for some new, interesting, and exciting cuisine, but what we got was a McDonald's hamburger that they took the wrapper off and stuck some literal bells and whistles inside of.