"If the UBOH were to fight my kingdom right now, the whole earth would probably be destroyed."
"Scary."
It wasn't exactly an exaggeration, considering that Nagan could just attempt to empower both Bluebell and Hopscotch with the bubbling madness that they experienced when fighting the Faceless God inside the Trace of Azathoth. Obviously, destroying a planet wasn't equal to stopping the apocalypse and rifts around the world from existing.
Although, it was certainly something that was attainable after the whole Virtuosic War gig was finished, if Nagan were to dispatch the entirety of the kingdom's force onto Earth, which would definitely be finished in a year or so.
But doing such a large-scale operation would definitely leave a vulnerability or two for the progress of the kingdom, and probably render humanity from evolving and conquering the apocalypse on Earth by their own hands, especially if the same thing were to happen again in the very far future.