Omega and Midgardsormr are not sundered because they came to the planet after and it seems (keyword seems, but going by what we know of the cause, this appears to be the case) to be restricted only to the planet.

Which, in retrospect, makes me wonder if Omega's aether-crafting abilities aren't really all that special. It's pretty much the same thing the Ancients did. Are all unsundered beings across the universe able to use summoning that way, and rather than Omega being super powerful, it's more that Hydaelyn-ians are weak?