I did some testing with Jab->Up-B on Dedede, and at least in his case it depends on what type of landlag they get for whether it connects. Normally he gets 6 frames of landlag after landing from the jab, and when the Up-B (hits on frame 6) is done out of the jab correctly (ends on frame 15), it hits him on that 6th and last frame of landlag.
However, if they press shield/airdodge before they land from the jab, they get a softer landing and can shield on frame 3 instead of frame 7 and so the Up-B gets powershielded.
Basically, at least in his case (and likely many others) you're more likely to miss when the Jab->Up-B is expected and they're looking for it, but they weren't actually planning on getting hit when the jab connects. In this case they react and press shield, but also are able to do it quickly enough to press it before touching the ground which begins a dodge and they land with less lag. If going off reaction they would need to react to being hit by the jab and press shield in under ~0.300 seconds.
If they are expecting to be hit (like say they did a laggy move) and are holding shield to preemptively protect themselves it will work if their laggy landlag allows. Also if you catch them offguard or while they were trying to do something else at the time it would generally work, since they would be far more likely to press the shield button after they've already touched the ground and are stuck with the laggy landing.
DIing to the side might cause them to land like 1 frame sooner than normal (I didn't get to test this), and in cases like Dedede's where it hits on the last frame of lag it might cause it to miss even if they do get the laggier landing. He is much heavier and falls faster than most characters though, so the others likely don't reach the ground sooner than him, and it'd probably only vary by +1/-1 frame. There's a much larger difference between the two landings though, which I imagine is the main reason it hits/misses.
Also, some characters may always get the softer landing after being jabbed even without airdodging, so in that case they would always be able to shield it so long as they press shield before the Up-B connects, which is pretty easy to do in time.
Oh, and if they happen to tap down and fastfall early enough during the brief period of time they're off the ground it may allow them to shield in time as well by landing a bit sooner.