Looking at the actual gameplay mechanics, especially with the low HP levels of JSawyer Mod, it's evident that New Vegas would be a solid shooter if the foundation (engine, AI, etc) wasn't what it was.
Nah, this is where Obsidian's monocle fell short. If they focused on combat, they could have elevated it to a greater degree gamebryo or not. Hell I may even prove it with another mod one day. I did a fair bit of AI work on Deus Ex as you know and the vanilla poopy AI was primarily limited by how much work was originally given to to it. The chosen engine has an influence but the main thing is simply putting in the time (well, and good design sense is of course required), which the devs have been on paper showing regret for not having done so, even though what they did do was already a shitload of work.
Yeah the base FO3 design and engine definitely counts for something, but I know it can be made notably better with more focus/time, and don't think it's inherently doomed as a result of being a FPS/RPG hybrid blend of real time skill and stat-based outcomes, the existence of vats, whatever...but then again I love FPS, RPGs, and FPS/RPGs, so of course I don't.
If you did an AI overhaul for New Vegas that would be insane.