Many powerful bananas makes a specialist economy really really easy, you only work the banana tiles and then get lots of specialists.
I don't think we want this. That makes them very strong.
It also means that once the jungles are cleared, you have a paradise, because you have lots of bonus jungle tiles *and* a bunch of regular plains.
Again, the main problem with jungles is that they're just as good for food as grasslands, but also offer science and pantheon belief bonuses.
Probably the best solution is to leave them at 2 food with rare bananas, remove the pantheon belief, and bump the science bonus to only +1, and only in the late game.
So, jungles have downsides (longer improvement construction time) but also upsides (more flexibility, small science bonus late game).