I find some elements of FO3's gameworld more belivable than NV, but hey that's my opinion. Now tell me I'm wrong.
This is kind of a worthless statement without stating what in particular you find more believable. Precisely how am I supposed to evaluate the merits of your opinion if you've made only the foggiest of claims?
In a broader sense, no one has argued that every single aspect of Fallout 3's world is unbelievable. Obviously there are people, and they are trying to survive, and within the context of a post-apocalyptic world this makes some degree of sense. The problems arise when you look at exactly how they survive (both in terms of where they are and what got them there). I can overlook things like the total impracticality of Megaton's construction. Maybe they really dig planes, maybe they're inventively stupid, it doesn't matter---Megaton is
possible, however unlikely. It's just lame world design, with little effort spent thinking about
how a world like this would work, made clear by the fact that you have to make improbable justifications for why thing happened the way they did.
(I'd say it's still easy to objectively measure the probability of a settlement forming one way or another. If nothing else, we can simply use occam's razor to identify what is a more believable outcome.)
There's a difference between that, however, and the total lack of infrastructure in most settlements. Take Dukov's place---what even is this? How can he possibly feed himself, much less his two guests? It's a two floor house filled with drugs and booze. He's wearing pajamas, for chrissake. I assume he's not getting out much.
This is a scene from a movie or a book: a survivor stumbles across a potentially dangerous hedonist who's resigned himself to a fatalistic party-til-you-die attitude. In that context it makes sense. A movie/book is linear and told from a single perspective. You don't need to know whether he's keeping livestock and a garden out back because that's not why he's in the story.
A game, especially an open world game,
does need to make these considerations. The player isn't locked into a single perspective. They can explore every inch of his house, they can check every cranny around the perimeter, they can press him regarding his situation. When there's no verbal explanation for how he continues to survive with a diet consisting entirely of six and drugs, and there exists no physical evidence that would suggest he has other means of procuring food, that is a failure in design.