There's a big difference between subjective and voodoo. By that definition an umpires strike zone is voodoo, I think all non cynical baseball fans would agree that while umpires can have a bad day and be a little off a strike zone is subjective. It might vary but it's pretty darn close, and like Moon said they have trained observers.
Even though we know umpires are imperfect we trust them enough to never blame the umpire for a pitcher with bad control. No one would ever say that we can't trust Henry Owens WHIP, and BB/9 because umpires strike zones are voodoo....you just wouldn't. Defensive metrics are very similar in this sense, people just don't see it that way because it's new and what is new scares people.