I have this idea that FO3 is designed with PC gamers in mind.
It's more tidy and carefully constructed.
Whereas FONV has lots of quality control problems that console gamers would never find.
Because a console gamer sees a mountain and thinks how can they get around it easily. A PC gamer sees a mountain sees a mountain and tries to climb right up it.
And so in NV, the PC gamer falls into the giant gaps in the world and into the abyss between dimensions.
The console gamer doesn't notice them.
It could, however, be a function of FO3 being the urban/suburban environment and therefore having less opportunity to climb mountains.
But it definitely seems as if textures and meshes cover what they need to cover in FO3.