A major deterrent could be the law, but also from the look of things today's "mobsters" in America don't even seem to be cut from the same cloth as those guys in Canada. But we also seem to ignore some relatively recent unsolved mob hits in America, like Ralph Coppola, Anthony Seccafico, and Christopher Castellano.
Three good examples of how you would have to do something like this:
1. Pick someone who won't rat (if you don't have this, then don't even bother)
2. No cameras and not in broad daylight (see Nicodemo, Anthony)
3. Castellano (wait until you can mix the shot to the back of the head in with a 4th of July fireworks display).
4. Seccafico (wait until the target is waiting for a bus at 4 am with noone around, must be very rare for a mobster)
5. Coppola (do a very very good job of disappearing someone, and make sure number one doesn't take place)
6. Don't ever talk about it to anyone (another difficult step in this day and age).
