You have to take difficulty into account. So Kirby is the easiest since all you have to do is stand there and up tilt and shield breaks are pretty common.
Ness and Yoshi are obviously a lot harder thus way more room for error (can you even mess up a kirby shield break lolol??).
You can mess up Kirby's up-tilt shield break depending how far they are from you. For example, if you're facing away from them and they're pretty close to you, you can have the ultimate u-tilt that is able to combo/juggle them if they don't shield and the full shield stun if they are shielding. If you're facing them and they manage to get hit by your u-tilt, it's kind of a light tap but you're still able to juggle them if they aren't shielding but if they are you still have a decent amount of shield pressure
but they can escape it if you're too far away and your u-tilt barely reaches them. So basically a Kirby can mess up the shield break if they don't space properly and realign accordingly because the u-tilt pushes them away. If you're in this situation, you can just turn around or pivot accordingly to get that shieldbreak.
I find Ness easier to shieldbreak than Yoshi. Ness and Yoshi have it bad because if you don't time it correctly, you can get punished and since DJC requires your second jump, you're ****ed if you get hit out of it. Yoshi has no recovery and well, we all know how horrible Ness is. My only gripe with Ness is that like Fox (shine cancelling), sometimes I DJC too quickly and no move comes out so I guess that's a pretty common error.