Yeah I've heard about this.
There are a lot of 'Russian' and 'Ukrainian' criminals who are ethnically Ashkenazi Jewish. Mogilevich is Jewish, Agron, Balagula, Nayfeld, Fainberg,...all Jewish.
But then we have Mikhailov, Kumarin, Timofeyev, Ivankov,...and they don't have a Jewish bone in their body.
There are a lot of Soviet Jewish crime bosses, sure. But there are also lots and lots of ethnic Russian mobsters. Like PolakVet said, a lot of ethnic Russian gangsters faked Jewish background just to hide in Israel. The 'Russian' Mafia is neither fully Jewish, nor fully Russian.

ps : I live in Belgium. Antwerp used to be a big place for the 'Russian' mob. The main bosses were Rachmiel Brandwain and Mosche Ben-Ari, both Ashkenazi Jews from Ukraine, and Abraham Melikhov, a Georgian Jew ( ethnically Georgian Jews differ from Ashkenazi Jews). In Belgium citizenship was applied to people of Jewish origins. Just like it in the USA. That's why a lot of Jewish crime bosses are around in these countries.
Nowadays these crime bosses aren't that active anymore in Antwerp. They used to run prostitution, drug trafficking, arms trafficking, contract killing, money laundering,...nowadays (the ones who are still alive) they only launder money.
Albanians in Antwerp mostly control drug and weapon trafficking and prostitution nowadays, but there also is an increase in Chechen gangsters over there. They have tried to take over some Albanian businesses which has resulted in shootings a couple of times.