Chronoblast's damage profile is skewed in the direction that you'd expect the HP of creatures coming out to be skewed, so it might tend to do just enough damage to kill. For example, if a green deck puts out a leaf dragon, 2 frogs, and 2 cockatrices in that order, it will kill the dragon, a frog, and a cockatrice, and leave the other creatures at 1 hp. This is stronger than what RoF would do in that specific situation. I'm not saying it is necessarily better than RoF, but it might be surprisingly good because it ramps in the same direction that a player would normally ramp the HP of the creatures he plays.