Sai had enormous power. He could TK and break peoples limbs, and punched out a dragon. He could create illusions and stopped blades with his fingers. And at the end of the anime he destroyed a colossal magic tree that bound their fates and basically reset their lives (which supposedly puts him in an even worse position the next time around, but gets through it.). He has also regenerated a destroyed limb with his magical power.
This is a kid whose biggest dream was becoming the Pope and doing good for the world, then told his future was to become the Maoh (Demon Lord), and pretty much everyone turned on him. To which he had to work just to get anyone to trust him, even when he couldn't control his own power at the start because it was so immense.