The Opportunist trait doesn’t seem to work, at least not in a way that is meaningfully noticeable. As a replacement for Devastating Blow, the most singularly noticeable effect of Dev Blow is missing - the ability to interrupt Stormvermin overhead attacks.
Testing on both live (1.0.4) and beta (1.0.5), and on Champion difficulty a buddy with 558 hero power and Opportunist on his 2h Hammer attempted to shove a Stormvermin who was in the midst of an overhead attack, and was unable to interrupt it. Same on a 2h Axe.
A shield without the trait interrupts them just fine, however, just like in Vermintide 1. This leads me to surmise that the effect of the Opportunist trait, if it is functioning properly, is substantially lower than the effect of Devastating Blow from the first game.
All of the weapon traits should feel like they are having a direct and regular effect in the game. This one does not (and neither does Heroic Intervention).