Well, if there was a good way to summarize how the move functions, it's sort of basically (not really) a vertical f-smash that's done in mid-air. Comes out at frame 14, deals 15% damage if sweetspotted and it has surprisingly insane amount of knockback if you land it. The landing lag of u-air lasts for 17 frames which is bad. This move also lacks a horizontal range or at least, its horizontal range is severely lacking, but it's vertical range is NUTS. It even outranges u-tilt's vertical range
U-air's main saving grace is the reward for landing it because holy
crap it deals a lot of damage and it kills at surprisingly fraudulent percentages
even without smash art. Its range is also deceivingly
amazing vertically. You can contest any aerial with u-air because of this. Anyway, the first hit of u-air has fixed knockback so if you manage to land this (somehow) without landing the second hit, you can combo off from the first hit. With MALLC, you can obviously combo off from u-air even if you connect both hits. U-air is a capable juggle tool but it requires muuuch more precision than u-tilt.
The issue with u-air though is that its frame speed is absolute garbage and it's hard to land unless you're in jump or speed art since you have the air speed to somehow land this but even then, it's still quite hard to land. Horizontal range is lacking, comes out too late (frame 14), and basically, it's horrible if you whiff it. Even worse than f-air since you have to be close to the opponent to land this or below the opponent but even if you whiff it below the opponent, they can FF and punish Shulk easily
With jump art, u-air becomes scary because with your jump height and air speed, you can throw and possibly land u-air out of no where and kill them from high up the stage boundary. You can also set up into u-air from jump art u-throw. Speed art, u-air is easier to land against grounded opponents thanks to the increased air mobility and shortened jump height. Shulk also has slightly better (but still bad) air acceleration which also helps, so yeah. Shield art, basically it's a worse vanilla u-air but whatever. Buster art u-air is basically vanilla too but it's a direct upgrade since well, sure, it doesn't kill but that damage is INSANE. Shield pushback and shield damage of u-air is also nuts, and that's what makes buster u-air actually not that horrible. Smash art u-air is disgusting. Hard to land, akin to buster and vanilla u-air, but the reward is arguably more insane since you can end stocks easily at really low percentages
Basically, u-air is a situational aerial. You won't find yourself using this as much as b-air, n-air, f-air, d-tilt, etc. It's pretty good for juggling, and that's one of its solid uses. You can combo with u-air but it's quite difficult to pull off a combo from u-air since you need to either MALLC or strictly land the 1st hit only. Hard to land, good reward, but you won't use it a lot, unless maybe if you're in jump art
普
1/2
翔
疾撃
1/2
盾
斬