Well, the stuff that happens at the end of the series is when she has an armada of robots under her command that do what she wants. She's not going to become a planet sized doom machine without them. As for slicing moons in half, that was extreme circumstances in which a space monster was going to destroy humanity. Otherwise, she'll generally refrain from using it. Kinda like that secondary ability Excalibur has that causes a massive explosion, or Starlight Breaker. (Also slightly nitpickey, the moon didn't explode. It just got cut in half. At least, iirc)