Why go after WW2 era when there are plenty sitting pretty in Zagred, Sarajevo, Tuzla, Hrvatksa and throughout Kosovo. Forget "WW2" era, no one cares about the past, get the current era whilst it is still freshly engrained into the current generations head. They'll remember the arrests and repercussions.
So that those "sitting pretty in Zagreb" have nowhere to go, nowhere to hide, no financial support from the old Ustashi in the diaspora to hide them on the run when.if someone does come after them.
I don't see this as an either/or situation. You go after BOTH. The difference is that when it comes to the old Ustashi, you have the support of Jewish organizations who have always been far more effective at mobilizing the resources necessary to really get these bastards nailed than the Serbs alone ever have been.