My personal issue w/ that second ball safety is it's cake to tick your opponent to the side rail and spin back to kiss the short rail (my starting position). I've always tried to be the first one to kiss the rail. Otherwise, it's too easy for my opponent to put me there (and I think it sucks from there).
From that position, depending on the angle, you can force someone to knock a ball loose by performing what should be a standard safety. Lots of bad things can happen for your opponent while touching the bottom rail -- and the only bad thing that can happen to them by touching the rack is being on a tangent to the pocket. Otherwise, the follow-up safety is a throw-in.
The back scratch is available; however, you always run the risk of a back-row ball sinking off the rack. If your opponent tilts the rack ever so slightly without you noticing, you're doomed. I'm not saying it's a bad move, it's not. I'm just saying I try to eliminate the chance of bad things happening while forcing my opponent to come into/off-of the rack first.
If my opponent kisses me into the rack and gets fortunate enough to tangent me to the corner, I'll intentional my way out of it.
Hopefully that makes sense. I just wanted to explain my thought process. I think my biggest strength is kissing the CB to that short rail. If someone wants to shoot at a loose ball at a funny angle up table while kissing the rail, I always let'm.