I believe this has a lot to do with your opponent's inputs, they can Smash-DI out of the attack (move their character during the hit "stun" of the attack). I do this consciously and deliberately when I am hit by that U-Smash to get myself out. Others may be getting out of the attack kind of on "accident" if they are inputting movement when they are getting hit by it.
Unless it gets "buffed" in the same way other U-Smashes did (Link's comes to mind) you'll just have to use it at your own discretion without expectations that all the hits will fully connect. Although there are positions you can put yourself in to help facilitate a full connection (hitting "deeper" into them will make it more likely to hit all parts).