Descending from a world was one thing. But ascending from a lower station to a higher one was a completely separate matter. Because Aina had the foundation of a Mortal, she gained much more leeway from Regulators. This was why Leonel hadn't felt any trouble entering the Ma'at Bubble despite it being a Mortal World.
As for why Incomplete Worlds still started them, it was because there was a dividing line between the two states of Complete and Incomplete. In addition, there was more being targeted than just their constitutions as well.
Incomplete Worlds couldn't withstand the Ninth Dimension, nor could they withstand Forces surpassing the Impetus State as well. So even if Leonel and Aina managed in one fashion, it was difficult to manage in others.