Advocacy documentaries are a huge risk — such blatantly opinionated content risks alienating viewers, playing to an already established audience, and, in the end, failing to educate anyone. Still, some manage to make a big splash, for better or worse (“An Inconvenient Truth,” “Super Size Me”), but others can truly be sickeningly partisan garbage (anything by Dinesh D’Souza and his manipulative ilk). It’s quite a wide spectrum, but how could it not be? The medium of film is undeniably powerful and people with money have always been eager to exploit it. But the far more mundane reality of advocacy documentary is that so many such films are middling, mediocre exercises that will never find an audience besides those already invested in the cause. Which is likely the fate of “Unfractured,” the new doc that chronicles the life of Sandra Steingraber — a scientist and leader of the anti-fracking movement.