I can't give you any statistics. However, when I took a mountain medicine course one time, the instructors taught us a thing or two on what to expect. The attendees were mostly state park rangers, Sierra Club backpack leaders, nurses, and a few physicians. They did not have it sorted for on-trail versus off-trail injuries. Obviously, off-trail hiking has more risks.
One of the most common injuries is when the hiker is impaled by a sharp/broken tree branch on a tree that is down across a trail. Maybe only on the leg, but it could be anywhere. Maybe not too much more than skin deep. The problem is that some thru-hikers are out on some trail without any assistance for a week or two at a time, and if a wound like that gets infected, there is a problem. Weekend hikers don't have the same infection risk, since they will be home in a couple of days.
I found this one strange, but we were taught that too many hikers have carried along some "recreational pharmaceuticals" and poisons, and we were given a long list of overdose symptoms to look for in an impaired hiker.
Obviously, when we are hiking along a trail for 15-20 miles in a given day, we pretty much have to keep it together.