My recent acquisition of two Rapido cars (PSC van and gondola), and a fleet of NARC coal gondolas were flawless. They all had many extra detail parts, etchings as well as OEM parts- complex beasts, highly prototypical and at a pretty reasonable price. The quality is out there, in my experience.
I have a Walthers well car set that the links (for lack of a better term) between wells would slip off and separate the cars because the pin was too short. A minor design flaw, but similar to the theme of the OP.
I don't think it is, in general, a big deal. I expect quality and have for the most part, received it with my n-scale purchases. I can complain with the best of them, but this is an area of my purchasing where I am almost always satisfied.
Ironically, I frequently buy scientific instruments for work that cost tens of thousands and some of them are basically prototypes that require intensive, often expensive remedies to be functional. I wish they all worked like a Kato locomotive out of the box!
