Bit late, but I don't think characters like Tiamat or Middy are so powerful because there Unsundered, I think it's just because there Dragons and Dragons are naturally freakishly powerful.

Were giving no indication that Mortal's from the Shards are inherently weaker then Mortals from the Source. And we have been given no indication that "Immortal boundless wells of Aether" is the natural state of all Sapients from other worlds.

In fact, even if you decide to ignore all the Crossover events, Omega still disproves the latter, because he is explicitly pulling from other FF games where we know the natural state of what would be the Unsundered equivalents of those places is not "Magic Demigods"