Will the 2x might be tempting, it will lose its novelty when it is compared to many of the rainbows, especially the ones with the 1.65x for all. Yes it is not 2x, but the rest of the abilities more than make up for the loss.
Afterall, a dead enemy is dead. If you kill it always with your first unit, the rest will not attack because it is dead. You just want to ensure the team as a whole can kill on the first turn. If you rely on the first turn too much, when fighting the higher stages where you will be expecting 4~5 turns to completion, you might get surprised :|