The one problem I saw with most of the mobsters was the lack of viciousness they had most of the time. The best way to describe it would be compare the crews in The Sopranos to Casino or Goodfellows.
That's an unfair comparison considering most of the time we saw Nicky's crew working in Casino, they were killing people or beating the hell out of someone. And a film like Casino was focused more on the violence and out-of-control nature of Nicky than The Sopranos, which was more centered around Tony Soprano's crew, family and his internal problems. The Sopranos was a show about a mob guy, but we rarely saw acts of extreme violence by anyone (even the hits were usually very brief).
Plus, the crews in both Scorsese films had very few redeeming characteristics as shown to us. Meanwhile, Tony's crew, despite the nature of what they do, came off as fairly likable early on, but slowly became unlikable over time. They didn't bludgeon you over the head with their violence acts on screen for the sake of shock value or to demonstrate how they handled their business with two-minute "let's show the crew killing random people in several random scenes" montages.