Like Coach, I think there are many variables, and we each have to decide for ourselves. I also sorta understand Shad's position, but Shad, you are in Spain with a totally different setup than most of us "backyard" chicken keepers.
While I incubate eggs in "proper" position, broody hens don't follow this, and they still have excellent hatches, so I'd have to disagree with any so-called study on the subject, because I've witnessed it with my own eyes many, many, many times.
I have had 2 or 3 hens kill chicks, I always give them another shot, and they all became wonderful mothers. Whatever circumstances caused them to kill was beyond my guess, but they were not doomed for eternity by it.