We only use manual steps, ranging from RJs to A380s. Common injuries I've seen are the usual pulled muscle / back injuries from having to pull or push the steps alone onto the aircraft.
One particularly nasty injury I saw... The ramp agent was pushing the 'handbrake' down, which was extremely stiff. He applied as much pressure to it as he could when eventually it moved, trapping two fingers between the metal bar and the handbrake handle. 3 broken fingers later his hand was released
Maybe not particularly relevant for the powered vs manual case, but a while ago the steps had a full load of passengers on them queueing to get onto the aircraft when they suddenly 'collapsed', dropping a meter or so.