You might not be a barner, but you said one of the most scathing things I've ever read here, about Saban, during the so called "Texas fiasco". Every now and then you let some bias show through... you're not objective on the subject, if there's a negative viewpoint to be had in regards to Saban you usually manage to have it.
First, I would say I think "slow and strong" as you put it can create more of certain types of injuries. There might be more mass involved, it has to stress certain points of the body more, etc... however, this has to be focused around the primary area of concern, which is head injuries. How they are caused and when they are caused is highly relevant. First, high speed plays are more likely to lead to the types of collisions that lead to concussions. There's a reason they put those rules in to try and protect receivers, that was the type of wide open play that could lead to a guy getting his "bell rung" so to speak. In the same respect it's one reason they have so much protection for kick returners. So, while I agree certain types of injuries might increase under a slower pace of play, clearly the types of hits that tend to lead to concussion are both kind of flukes and increase in probability the more plays are run, but they also can very easily happy during fast, sloppy play. Remember, there's an issue with women soccer players getting concussions. Size and strength is not the main factor.
In either case though the sheer quantity of plays will increase the health risks overall. Your point about subbing is interesting, since that's one of the major issues I have with the HUNH, the inability for players to sub in and out. The problem is, if someone is hurt, or might be hurt, it takes a bit of time to ascertain that. The coach, the ref, even the player them self might need a few seconds to figure that out. Is this pain going away? Is this pain serious? Is he hurt? Do I need to stop the play? All those questions are made to be answered in under ten seconds in some situations (they did after all block the ten seconds rule). There's no way that's not a risk to the athletes. If you're an athlete and you know you have to run straight to the line of scrimmage after the play, if you're limping, or what ever, you probably just run up there anyway. You can't be out of position while you try and figure out if your hurt bad or not. The refs and coaches aren't able to pay any attention either, because they're tripping over themselves to worry about the next play to. If they dealt with that issue properly, it would alleviate a lot of the concern I have.
But, the entire HUNH approach is a dangerous one in the long run. Players not getting set, players not being able to sub out, running as many plays as possible, a style of offense designed to push the other team to the point of exhaustion without letting them exit the game, and something that is built around distracting and confusing the players, all those things pose threats. Think about this, if a defensive player is focused and where he needs to be, well rested and so on, sure he can give a player a good lick. But, he knows what types of hits he can't do, however, you take the same guy, you confuse him, you distract him, you wear him out a bit, he might get lazy and tackle a guy wrong, or he might just get frustrated. Either way, the coaches are now coaching them to tackle in such a way as to not cause head injury, confusing those guys as much as possible seems like it could undo some of that coaching could it not? So, I mean it's a complicated issues but looking at them piece by piece I simply see no way to arrive to the conclusion that a slower pace of play is going to lead to more permanent injuries (which I believe is the relevant point here).