Have you seen high level play of Merkava, Waldstein, Yuzuriha, Hilda, or Carmine? Even a character designed around mobility like Seth (who also has near full screen special attacks) is barely able to out manuver their options a majority of the time. Air mobility wouldn't even be a big deal because of the unblockable ground to air moves. Uniel was intentionally designed to give characters a stronger ground game compared to air game in order to make jumping in a risk akin to early street fighter.
I may not be extensively familiar with every single Arksys character but I have seen tournament footage and played all three franchises. Under night characters come off as the strongest to me; but hey, if they stay mostly unchanged in cross tag then I guess we'll see for sure next year.