From Wikipedia:

"A major theme of palantír usage is that while the stones show real objects or events, those using the stones had to "possess great strength of will and of mind" to direct the stone's gaze to its full capability.[T 2] The stones were an unreliable guide to action, since what was not shown could be more important than what was selectively presented. A risk lay in the fact that users with sufficient power could choose what to show and what to conceal to other stones: in The Lord of the Rings, a palantír has fallen into the Enemy's hands, making the usefulness of all other existing stones questionable. Commentators such as the Tolkien scholar Paul Kocher note the hand of providence in their usage, while Joseph Pearce compares Sauron's use of the stones to broadcast wartime propaganda"