At least, that's what I've heard. It's because the babies don't "sit" right on them, but rather "hang" like in a parachute. Especially bad if they are looking facing front. Plus, they say it's really bad for the person carrying too, because it doesn't hold on to the hips like other slings/ergonomic backpacks.
But I get to see so many in London that just thought, maybe they are not so bad after all? Are parents really aware of this? Why are they so popular?