[@rechonq] Of course it's OP, it's very OP, but while Asgard used to be a big bad powerful realm it no longer houses an entire pantheon strengthened by mortal belief. The pantheon was deposed and the followers forgot long ago. Olympus is the only place with true power anymore, aside from the Underworld. Not many live in these other realms, and a handful of minor immortals living there doesn't make up for it. Olympus is home to most of the Merged, with others living in the Underworld or on Earth. Technically it could be destroyed as well, but it would take a lot of the Merged dying and a significant boost of power to the weapon in order to try it.

The Underworld is too big, there's too much to it, it can sundered in chunks, possibly, but only after significant cleaning out. It's unlikely they'll even try.

Earth is the only totally safe place, it's the only true original world, and even then just because the Old Gods aren't there yet doesn't mean they won't come hunting, but at least they can't destroy it using their weapon.