Most of them are crap.
there, I said it.
Rarely do they work in a story, unless you are a master in characterization, pacing, and character motivation.
the Authors just antagonize their readers with either all-knowing baddies, "look-how-evil-I-am" beard-strokes or spoiling future plot points that the MC doesn't know of yet.
Or they are so badly integrated, that the MC just looks incompetent while the reader knows all the secrets.