The hit on Sonny with automatic weapons was for dramatic purposes. In the novel, he is murdered with handguns.

Barzini only had one bodyguard if he had any. His driver was waiting in his car. The other guy with him may have been his attorney and was probably unarmed anyway. There was nothing sloppy about the hit. Using a policeman's disguise would have not alerted anyone. Castellano was murdered at drive time in the middle of Manhattan and with pistols. There's been any number of dons murdered over the decades and many if not most in broad daylight and without much difficulty. The range from which Neri shot Barzini given his experience would have made the shots easy.