I say, whoever wants to play Rainbows in Multiplayer, do it. More power!
But to those who already have and will in the future, don't ask why the nation with the SC is much larger and stronger than the nation with the Rainbow.
SC tends to have less research than RB, especially if SC is out playing war instead of studying at home. If RB chooses to use study to develop something that can stomp indies about as effectively as SC, then expansion may be nearly as good for RB, plus RB has a big head start on research.
All kinds of styles can work, or not work, depending on what you do and don't do with them. A RB mage who doesn't have a good plan, or who lets himself get killed, can be a lot worse than a SC, whose plan is pretty obvious. But there are some good and potent plans for RB's too - they're just less obvious.