Depends how you define unrealistic. Soprano family is indeed based on DeCavalcante family in terms of location of family, the size and some of the rackets they ran, but other than that the members of this family are fictional characters. That doesn't make it totally unrealistic otherwise pretty much any work of fiction is "unrealistic" because it didn't exactly happen this way in real life.

You have to accept these characters as real and as long as their story is realistic in the context of the environment they're in, if the interaction between them is realistic and so on, everything is fine.

Is anyone from the characters in the Soprano family unrealistic as a mob character? IMO nobody, they all pretty much fit into "the life". The difference is that they are more violent than the actual DeCavalcante family was, but violence as such is not unrealistic at all considering that this is a series about mob.

The number of killings has indeed decreased, but this doesn't mean that murders couldn't have happened this day and age if leadership of some family wanted to resort to violence. All it takes is the boss giving the order to one of his soldiers. It just happens that modern bosses aren't ready to do this for a number of reasons, but since when it's "unrealistic" for a mafia boss to order killings and for mafia soldiers to commit them?

Tony's character was paranoid, selfish, violent and had problems with rage. You had tons of mobsters who were like that. It would actually be unrealistic if there was too little violence. He lead a small but deadly family with a lot of soldiers who were prepared to kill and did commit murders through series (Chris, Paulie, Sil, both Baccalieris, Gigi, Patsy, Benny, Walden, Eugene... Hell, almost everyone from the major characters, even Vito was doing hits) not to mention his connection with Naples. You can imagine him as some sort of modern day DeMeo in this regard.

This also explains why even that large NY family feared him and didn't want to try to discipline him until Phil became boss. They even called the NJ family to perform hits for them. Especially Lupertazzi was your typical modern day non-violent boss - with the example of him and his family the series actually showed us that violence of Soprano family is not the norm these days and that even big families are hesitant to use it.

The Commision appears to be de facto non-existant so there was no one to discipline the Sopranos and tell them what to do. That's why Tony got away with seeing a shrink and a number of other things and could pretty much do what he wanted on his territory. They still made a lot of money with him and that was the most important. Again, totally realistic.

You have a real life example of that Albanian gang (of pretty much the same size as fictional Sopranos) dissrespecting Gambinos and there was no retaliation.

Another thing you have to consider is that most of these murders were murders of low level associates (or not connected at all) who were probably under the radar and their murders couldn't be directly linked to top LCN guys, for example Brendan, Mustang Sally, those two guys working for Chris, Jackie jr. and his guys, all those civilians etc. I believe such murders happen in real life too, but we simply can't know about every LCN-related murder/commited by LCN member. Not that many made guys died.

I think through the show we can easily see that Tony is not a typical mob boss and his family is not a typical mafia family either, if we're comparing him and his guys to Lupertazzi family who resemble modern LCN more. Creators of Sopranos didn't want to present modern post 00s mob in documentary-like fashion but were presented us what could have happened in a modern mob environment with a bit different set of characters than actual NJ mobsters (but still in the realm of general character traits of mafiosi).

In terms of realism they did far better job than say Godfather IMO. It's unjust comparing Sopranos to Godfellas and Donnie Brasco because these two are based on real life stories. Out of fictional mob stories Sopranos are the most realistic.