I maxed shield bash 15/15, tried it out in druid circle for over an hour, literally spaming it. Every single time it resisted. So I figured, ok... maybe there is some sort of mob, or level restriction. So I port over to highshore village all giddy ready to see some stunnage only to see this at the top of my screen "the friendly puppy resisted". Needless to say, my shield bash is now 1/15.
So now we know the chance of it's effect is consistent and sucks. Still, is its effect useful? Does it bring down the opponent's attack enough to make a difference? I guess we will have to test it in PVP for the final conclusion.
I will be testing it at 15/15 when PVP comes out and will ask my target about the decrease in their attack stat.