It must be frustrating to go to an event, shoot a lot of pics with what you think is good equipment and return to find disappointing results. I sympathize with you there.
The only question you ask in your post is this: "Is the 24-105 really as bad as people say it is?" In response, I have to say I never saw this as a general reputation for that lens. As far as I know, it's a solid "L" lens with a pretty good reputation. Obviously, it's a lower end zoom so you're not going to get the same results you might with a good prime or with one of the high-end zooms (new 24-70L or the newest 70-200L).
If you owned the camera and lens you used there may be fewer variables. As it is, there are too many variables to suggest why you did not get the performance you hoped for.
I don't know anything about the company you rented the equipment from. Personally, I know I can trust anything I rent from LensRentals, but I haven't dealt with other companies. I'm guessing they probably did not do AFMA on the lens/body you got. I'm also guessing you did no AFMA on the equipment.
You mentioned using AV mode at f/8 through f/11. While you said "good shutter speeds" I don't know what that might be. You've never said what the actual shutter speeds were. My experience suggests shooting a fast-moving event, even in good light, would require lower apertures with shutter speed at least 750, ideally 1000 to be safe. You seem to suggest the problem is simply OOF, but I'd check to see if there is motion blur at play here.
I've been using a 24-105 on my 5D3 this year with adequate results overall. I've found it gets too soft for my liking above 70mm, but I also haven't done AFMA. It's easier just to use it like a decent 24-70. When I want to be sure of tack sharp, I go to the Sigma 35 f/1.4 or the Canon 135L or even the 70-200L.
I suggest if you're going to rent in the future, perhaps the new 24-70L would be a better choice. And you may want to talk to your supplier about how they match bodies and lenses -- can they do AFMA for you? It's not cheap to rent such equipment, and they have a responsibility to make sure you're satisfied.