You will never keep the bar in the same grove as you would squatting in a contest. Another important factor. Most of the people I see box squatting don't really have a good understanding of the movement. You do not bounce of the box or rock back then forward while sitting on the box. We have never had a back injury in our gym from box squatting, or any of the other exercises we do.
Any exercise performed with bad form can be a high risk exercise.
I brought this question up myself in a recent post, but as far as I can tell, no one responded. They are referring to relaxing the extensors, if I'm not mistaken.