Kurins and O'Brien's "Boss of Bosses," leads the reader to believe that Castellano was a good businessman but a poor leader. Gavano said, contemptuously, that he was a "racketeer, not a gangster." His deal with Dellacroce kept peace (temporarily) in his family, but effectively split the family into the white collar and blue collar factions. He appears to have been unable to stop drug trafficking in the blue collar area, which got him into trouble, I guess the ultimate measure of his failure as a boss is that Gotti was able to have him assassinated with the concurrence of most of the people under Castellano.

It's not clear what his actual title was before he became boss, but it helped that he was Gambino's brother in law and cousin, Dellacroce was the official underboss.