Power means nothing without control.
Magnus had no control. (See what happened to him?)
Ahriman had no control. (See what happened to the 1k Sons?)
Rogue Psykers have no control. (Being conduits, rather than self contained individuals)
Eldrad has control. (Of most of the galaxy
Being able to destroy a planet at will is nothing compared to the power to alter the future so that an individual/race never existed. You would never have a "1v1 Psychic Duel" with Eldrad, because he would already have manipulated you into being possessed by Chaos/dying on another battlefield/being in another part of the galaxy.