The thing is Rosalina has good air speed, falls slowly so she can drift in and out, and her jump is good. Most characters have to get right in to hit anything and end up throwing a hurtbox and hitbox out. Weapon users have disjoints, but Shulk's Uair isn't fast compared to Rosalina's and he's not as tall as her which adds onto her ranged Uair.
Her Uair cannot be challenged from above and few characters can even challenge below them. Yes, you can challenge her from the sides and from below, but the question is how do you get there when she throws out a disjoint like that? Can you air dodge her Uair? Absolutely, but what if she reads it? You're going to get hit. You could jump or drift out as well, but she'll follow, especially against fast fallers or characters with below average air speed. In theory, anything can be avoided, but in practice, not so much.
The issue is the balance between speed and power with her Uair along with how she works that makes it strong. Anyone can kill anyone early on Halberd, but it's the ease that she can that makes the difference. Fox would have to find a way to setup the Uair and Uair again closer the ceiling at low percents, but his jump isn't exactly good. Can he run around and position himself? Yes, but once he commits, he's going to have to fast fall down since his air speed and fall speed won't let him drift around like Rosalina. Yoshi can run around and drift around too, but his Uair's range is short - of all the moves where his tail doesn't stretch. Rosalina has ground speed and air speed to move around. Once she launches you, you're at her mercy. Her Uair isn't broken or a super versatile move like pre-pre-patch Diddy's Swiss Army Knife Uair, but it's a bit too strong when you tie in everything.