My main problem with rainbow is that it has no strategy in place, it merely adapts to the board.
When you decide to stack heroes, whichever format you pick, you set a strategy in place: I will prioritize this color over the others, then this, and so on. I will try to get these heroes loaded first, with the fewest possible moves and least possible impact on my opponent.
In a rainbow team, it’s all about playing the board. I’ll combine whichever color is optimal for me, irrelevant of the hero in my lineup.
Furthermore, I don’t get the benefit of a rainbow team. In 3 turns, a rainbow team will definitely get one hero charged (the one favored by the board), but that’s still just one hero. In six turns, unless you have created a massive cascade, you’ll have two random heroes fire and three still halfway through at best. You should have at least two heroes loaded in six turns if you stacked too, and you’d have the benefit of tile damage, elemental link, and focus on specific color tiles.