Remember how, way back when, people would splash Emperor Nero into decks that otherwise ran little to no grey? He was pretty strong, and was more or less an autoblock if there was no banding (and Balaam's Disobedience tended to be thrown in in case there was).
Red Dragon is like Nero on steroids. Sure, he's not unbeatable, but even if they get around him it burns their resources.
Obviously, he synergizes best with Revelation demons, but he's splashable into just about anything.