I don't know what you guys are getting so freaked out about.
There is a shield at the rear that it pretty much dragging the ground and there isn't any debris coming out towards the operator.
You can see at the beginning that there was a gap between the ground and the shield, but when he adjusted the rear roller, he eliminated that gap.
Looks like a perfectly safe attachment to me.
My concern would be that the nose of the walk behind would drop, from hitting a rock or other solid object, and the flail mower raising up as I stepped forward with my sneaker clad foot.