Oh dear, I think maybe this discussion has reached the end of its usefulness?
And I didn't even get to talk about the roof-mounted lasers of ultimate destruction.

lol
This too. Always wondered what happened to those.
At any rate, there is ZERO in game evidence that the NCR or Caesar's Legion would be able to kill Mr. House. There is evidence saying his defenses would be impenetrable. Hell, even Yes-man's ending seems to imply that Mr. House had an ace in the hole for if someone did manage to kill him...
And to flip the argument Rook is making, who's to say that Mr. House wouldn't be able to bide his time long enough to think of a plan? The man is an absolute genius: he's the founder of Robco, he successfully predicted the Great War and defended the Mojave from 70 nuclear warheads and he's over 200 years old thanks to nothing but his own knowledge of medicine and science. He also shows that he can detect and delete a FoTA "virus" in 13 seconds flat. The FoTA, btw, definitely has a better understanding of science and technology than the NCR. You don't think the man would be able to think of a plan B in the time it takes the NCR to get in? Correct me if I'm wrong, but the only intelligent citizens from the NCR in the Mojave are the head technician at the Dam, Holden and Keeley. No one else is outstanding.
And what about Yes-man? A FoTA successfully reprogrammed him to become what he is now and they reprogrammed him so that Mr. House SEEMS to be clueless to his existence (correct me if I'm wrong, I might be), and yet Yes man is incapable of penetrating Mr. House's database. He can't jump to a securitron inside the Lucky 38, he can't access any info from Mr. House's database and he can't sabotage any of the functions of Mr. House's network.
Maybe he could rig the lasers to rain down on the soldiers, or he could give the robots a self-destruct feature so that they could suicide bomb the soldiers.
"But you have no evidence of this being possible." Yes exactly. Just as you have no evidence of Mr. House's defense being penetrable by the NCR.