The one I used to use at Heathrow had hinged flaps as the flooring at the entrance so that if you caught a foot over the edge it would lift up with your foot ameliorating that second risk. I think the main danger of serious injury (rather than a bruise or sprain) was people carrying overlarge objects into the paternoster. I was in the building when one snapped a ladder and it was pretty noisy.

The overall risk is low but probably higher than that of a conventional lift although paternosters should be far more efficient.