Barnes was only allowed to run a pick-and-roll every now and then, because the Warriors wanted to see if he could (at that point he really couldn’t). Durant was allowed to do whatever he wanted in the offense. Barnes would only get the ball in iso situations if he was playing in garbage time or if he wanted to get benched.
Durant can spread the floor like Barnes (again, I feel really silly typing this), but he is so much more it really doesn’t compare. CP3 can obviously spot up in the corner (or anywhere, like Beverley), but he will hurt a defense in many more ways when he’s off-ball.