I don't see how rainbow is overpowered. IF (a big if) every draw is optimal, it is better. But the quanta are random, just like the cards in hand. With a no brainer mono deck, you don't really have to think (and actually you have little choice) what card you play.
With a rainbow, you have options and also a bigger luck/knowledge factor.
And honestly, single/duo rush decks in TOP50 AI beat me harder than FG's.