From the point of view of god-killing, Rainbow is definitely overpowered. There is no deck that can match the potency of Rainbow vs-ing God, and its the deck that gives you the most wins.
What I'd like to see is more decks being made viable to kill gods, even if Elemental Mastery is not possible with such other decks (another gripe which I have about Mono-Rainbow arguments, but that'll be discussed some other day). Allow Mono-Pillars to produce 2 Quantums per turn unupgraded, and implement more Element Cards that requires a moderately high casting-cost and activation-cost. In doing so, Rainbow deck remains viable, but it gives Mono Decks a fighting chance both against god-killing and PVPing. There should be no reason why an unconcentrated multi-color deck owns single-coloured decks when single colour decks should, by right, produce a higher consistency rate.
The reverse argument would be to nerf the Gods instead, but that won't be happening anytime soon because the reward of god-killing is too great and every player is farming the gods everyday. The only thing in the player's mind now is "must get that next upgrade", which really shouldn't be the main focus of any card game in the first place. But the fact that it is the sole objective of the players makes it impossible for the developers to do anything to weaken the Gods.
Card games should be about deck-building, and trying as many combinations as you can imagine, and see how effective it is in real play. The current state of the game neither encourage nor allow for such plays, which is a really sad thing to see.
The way it is now, people farm gods day in day out with almost identical Rainbow decks. Only a small card pool will ever see play in a player's deck. It makes other cards completely and utterly useless. The problem will not be solved by introducing new cards, as players will then hop onto the "new Queen", the "new Sundial" and the "new Hourglass", abandoning old cards and replacing them with the better version.