I will agree on the general effectiveness of birdshot
At very close range. I think it is a fine load for inside the house use in an emergency. As to the lethality of bird shot, I don't care. It is often very lethal, but in a defense situation, I don't care one bit about killing the attacker. What I care about is stopping them. If they die as a result of being stopped, they die. But killing is not my primary concern.
A stray pellet or two is unlikely to be lethal, and may not even cause serious damage, but the full shot charge (and wad) certainly can. I have read many folks on the Internet discounting birdshot as a viable defense round, always claiming the lack of penetration as "proof" of its inability to be a good stopping round. If you have a large house, and fire across the length of it, you just might get a shotgun pattern to open up enough to start to lose some effectivness, but under 40 feet (and in my house that is farther than any distance I can shoot) the shot column hits like one large bullet, no matter what the shot size.
Those idiots who tell novices that they need not aim ought to be beaten bloody, as this "advice" is not only incorrect, it is potentialy dangerously harmful. Patterns do open up, but not instantly, and even the most open choked gun will only be 4-5 inches spread at max inside the house range. You MUST AIM a shotgun! You just don't aim it like a rifle or pistol. But you still need to aim, or you WILL MISS!
All else being equal (and it almost never is) bigger bullets tend to work better.