In this video we attack 3 bases with only 40 balloons. We experimented with balloons only, balloons with a hero and tried to attack a th9 base with our th8 balloons.
Your main enemy with balloons are wizard towers and high level air defenses. The wizard towers will do a lot of splash damage against groups of balloons, because they are slow they get shot a lot… Also air defenses tend to take balloons out fast, but they attack only 1 balloon at a time. Therefore balloons can beat air defenses with minimal loss.