Well of course machinery will mess up; but so will human beings, especially if you're doing large scale slaughter. You know how your wrists and hands get tired after cutting a few large carrots or so? Imagine slaughtering a hundred chickens.

Eh? We are talking about the critter that runs around like a terror after you've decapitated it, right?

To me, all that matters is that animals are killed in the quickest, most painless fashion possible. How gruesome that is doesn't bother me. From what I have heard thus far, the mechanical methods are not necessarily aimed at painlessness, and they aren't particularly accurate (Yes, doing it manually will result in mistakes after a while from the most experienced slaughterer, but that's where shifts come in handy). Until such a time as that can be improved, I will be against mechanical slaughter.