Fallout is bad from even a soft-SF perspective in the area of worldbuilding if you try to make sense of it rationally. Just about nothing in it stands up to rational scrutiny. But it's not intended to be that: it's a rule-of-cool pulp world that expressly references pulp aesthetics and themes. It's a "loving parody" of pulp, but I think the issue that Bethesda's creatives ran into is that there aren't that many people now who know anything about pulp -- same with the audience -- so it became very self-referential. You see this also with Star Wars and its later adaptations in which the twist on pulp became self-referential, squeezing out the vital and original connection to other bodies of popular art.
FO:NV was a little fresher because it reached a little farther with its references and aesthetics. I wouldn't really use the term worldbuilding to describe what it does well. It's really "set-building" in that, like a tightly directed play, all the scenery is relevant and ties into the plot about the Hoover Dam, which if you think about it is an appropriate reference symbol to the pulp era of the 1930s-50s. The 3D continuous world really plays well into this, pulling off a sense that wasn't possible with the first two FO games.
FO3 is really constrained at that in part because of all the technical compromises they had to make in the overmap design -- a city known for its open vistas and monuments became all chopped up with invisible walls and debris because of console constraints. Also, for whatever reason, the plot has almost nothing whatsoever to do with the primo location that they picked for a game. The big locations are just there to say that they were modeled in-game and don't really have much of anything to do with the story or the gameplay.
I think if they wanted to do a good Fallout, they would be better served by just plagiarizing Robert E. Howard as much as possible, embrace sexy pulp cover art as references rather than huffing the farts of its own IP, then it might do a little better. But if it follows the trend of things like the Outer Worlds, which again tries to draw from the overdrawn well of pulp, but then making it 1970s Soviet Dumpy in its desexed aesthetic, then it will continue to wallow around in bestselling mediocrity.