But (and it is a BIG BUT) I am willing to bet that at least half of the tech issues cropping up are due to running the game through Steam.
Why can't the developers just use the old tried and true method?
I guess you have absolutely no idea how Steam works, do you? Valve does not at all touch the Source code to the game... they just distribute it. There is no way Valve could induce "tech issues" into a game they are distributing other then when Steam happens to go down or something like that.
The fact of the matter is Obsidian is just a flat out terrible developer when it comes to game quality, as they have shown with every single game they have ever made. Sure New Vegas is a good game story wise, but any half competant developer should be able to make a solid game (mod) with Fallout 3 as a foundation. But on the tech saide of things, this is just par for the course when it comes to Obsidian. In fact, they even have a history of taking a fairly stable game/engine and making a technically faulty sequel in Knights of the Old Republic 2. So don't try to blame someone other then the people responsible. Steam might have its issues, but they have nothing to do with Fallout New Vegas being a technical mess.
Oh, and developers don't use the "tried and true" method because more people buy PC games from Steam these days then they do from the store.