Rainbow is flexible enough that there are lots of reasons it might be good, but its raw numbers aren't such that it's guaranteed to always be good. Anything which benefits from varied orbs makes it better. Any specific reasons to care about the orbs it channels make it better. The fact that it exhausts might be useful if you're hoping to play a smaller deck built around a few specific cards later in the run. Lots to think about for a fairly straight-forward card.
