The only reason that rainbow would have a problem with the cost is due to Otyugh.
If there was another way of killing creatures as efficiently as Otyughs do, that mana cost issue would not be one at all. Think about this. Does Rainbow need to kill every single permanent the opponent controls? No. Rainbow needs to kill the threats, such as some FG weapons or shields.
Plus, here's a little secret about these buffs and nerfs:
Know why people will only play mono color, Main color/mark supported splash, or rainbow decks?
Because the mana situation in Elements FAILS in terms of game design. Know why you saw all of these two and three color decks in M:tG? Two reasons:
1) Most cards had a vast majority of their casting cost in generic mana. So for instance, let's take a relatively simple card, such as Maxwell's Demon. In Elements, you can only pay Entropy for that card. But in M:tG, that card might cost two entropy, and 3 of any other mana you wished to use.
2) Mana fixers. Or their utter lack thereof. In M:tG, you had all sorts of lands that could do one of two things at any given moment, so it was like playing two cards for one. Rainbow simply uses the law of large numbers to balance things out.
Once you can have more control over how to direct your quanta, all of your mana cost nerfs and buffs will be utterly meaningless, since then Rainbow decks would be tweaked like crazy to support quanta they need to make more use of (life, time, gravity, death, light), and go lighter on the colors they don't need (Aether, Darkness, Water, Air (if no Eagle Eye), Earth), with Fire and Entropy falling somewhere in between (3 upgraded firestorms = 15 mana, entropy only needed for supernovas and to play druids)
Furthermore, Rainbow isn't massively good because some card or another is hideously overpowered. It's good because it can create synergies that simply should not exist because it combos a zillion color combinations together in ways never intended. And in a card game with hundreds of variables and an obscenely large number of possible combinations, there will always be a best something.
And so long as there is a best something, players will find a way to use it, and will sacrifice other less good somethings in order to make it happen.