I know they've always said, special teams have a higher injury rate, but I think that includes all players. Not sure about returners specifically
I've never seen a breakout of returner injuries. As you say, the stats available are for "special teams" where injury incidence is high leading to the stream of rule changes.
We're left with anecdotes such as the following, which are not a lot of help:
http://www.sandiegouniontribune.com...turn-injury-patriots-receiver-0912-story.html
That leaves the eye test and the nature of the job. There are guys flying at the returner from all directions, making him vulnerable to blind side hits where he cannot protect himself. There's gang tackling with low-high hits. On punt returns, there's the risk that a tackler, for whatever reason, does not honor the fair catch with the returner getting lit up.
The more aggressive the returner, the more likely he'll be injured, such as not fair catching with limited free run or a returner who cuts back rather than working to the sidelines.
I'm not sure returning kicks is more dangerous than running inside routes. On those routes, the receiver has to trust his QB is not going to lead him into a blind hit but that's not entirely preventable. I wouldn't be surprised if the incidence of concussions is higher on catches out of the slot.
Perhaps it is better to look at it in terms of probabilities: the more dangerous jobs a guy is given the more likely it is he'll be injured.
The fact returning typically falls to young players and specialists, with second contract starters far less common, says something. Amendola loves doing it which is the chief prerequsite. As for most established vets, whether by coaches decision, or player preference either explicitly or by evident business decisions on the field, they drop out of the returner roles.
Yes, all positions and roles on the football field are dangerous, some more than others. Returning kicks may not be the most dangerous but it's likely higher up on the list. Again, doing those jobs just raises the probability of injury.