The thing with rex is that he makes the oppoents pray that any kind of AoE does not crit, if they cast it at all. Current palp meta is heavy on the AoE, especially with Vader. Rex lead DN with Sion is sure to have a full DN turnmetter bar if vader crushes. So unless that palp gets reaaaally lucky, his AoE just might end the game, not even going to consider Vader crush, if you are using that opener against rex or DN Sion then that person isn't even top 200, at best.