The con is that it breaks cameras. Black Rapid straps disconnect and drop cameras on the pavement. Not every time, not every trip, but if you use one frequently it's going to happen eventually. It may be more likely to happen with heavier lenses and cameras.
It may well be that Canon cameras and lenses (likely other brands too) just aren't designed to be hung from the bottom like a Black Rapid, Carryspeed, or similar strap are designed to do. I've yet to find one that doesn't break and/or drop cameras. By contrast I've yet to see a strap attached to the strap eyelets at the top of the camera or side of a lens detach or break. The ergonomics aren't great, especially for larger lenses slung over a shoulder, but at least this doesn't break cameras.
What you are referring to is the original design which included a 'D-ring' style of attachment fitting that screwed to the camera body or lens. There were some reports of the fitting coming loose but once the issue became known, Black Rapid changed the design to a one-piece machined part which I believe corrected the problem.
From April to October, I spend almost every weekend shooting motocross. I'm on the track 6-8 hours per day, shooting with my 70-200 f/2.8L IS II mounted to either my 1D4 or gripped 5DIII. As mentioned above, it balances nicely when the strap is secured to the tripod collar. However, in that environment, when I am not actually shooting I prefer to have my gear hung with the front element of the lens facing down.
I have used the RS-7 mounted to the bottom of my 1D4 or gripped 5DIII with the 70-200/2.8 attached. I have had absolutely NO ISSUES or PROBLEMS with either.