Thanks. I guess I was thinking, in a more general sense, is there anything that governs or limits that power? I would think she would be their most powerful weapon otherwise.
In the books it's a difficult power to wield and doesn't seem incredibly strong. Its primary weakness is that it pretty much can only be used towards one person with one command at a time, and only if that person can hear you clearly. If his buddies are with him then once they realize what's happening they'd just slap the shyt out of him and say, "What are you doing, she's using the voice on you!" and snap him out of it. So it's more like a weak, temporary hypnosis than mind control or anything, you have to use it either in complete isolation or for very immediate shyt before the others can react.
There's also a strong impression that abilities like The Voice are hated and distrusted by most of the world outside of the Bene Gesserit. They're basically considered witches by many with all of the negative connotations that entails, so while many wish to ally with them due to their power, if they stepped out of line and used The Voice with the wrong person or in the wrong situation they'd get fukked up.
That doesn't change the fact, of course, that the Bene Gesserit ARE incredibly powerful. But they know their limits, avoid taking sides as a group, and stay in their place so they're not persecuted for being too powerful.