My girls love bananas and I've looked up on it, I haven't read anywhere that they're not safe. Some Wellness treats have banana in them too (Wellbar Yogurt, Apples, and Bananas), so I"m sure they're fine.
However, when I was looking up foods that are safe for rabbits, I found out that bananas can be addicting to rabbits, as well as Mediterranean Tortoises
. I couldn't find why though...