dWAR isn't perfect, but because talented players don't score well isn't a flaw.
If you look at the base stats for Bradley, he just isn't as involved defensively. He's second in all of MLB in innings in CF, but he's seen fewer plays than several other CFs. And his OOZ plays are middle of the pack. If you don't make as many plays, you don't score as well. Just like if you don't get as many ABs, you don't get as many hits. But that doesn't mean you're a bad hitter...