if they're facing you.
It's not great straight on because one: Who's going to let you just short hop and shine their shield in the first place, and two: jabs and shieldgrabs are much more viable options against it.
As a cross-up (cross-up meaning you hit the back of their shield), you eliminate those two options and it's must safer to try. D-air is an aerial that's often easily shieldgrabbed, but when you hit the back of a shield it can't be punished with a grab or jab, so you can start a sunshine combo afterwards. What sasukebowser said is correct; they can just drop their shield and take the 10%, but it's pretty much a win-win situation either way as a cross-up, because they either break their shield or take a hit from the shine/nair/jab cancel (unless they're MK, Marth, Diddy, or other characters with good OoS options).
What really bugs me is the space of time between the shine and N-air. Apparently (according to the framework thread)shine hits on frame 3, ends on frame 20, and N-air hits on frame 4, so there's 21 frames minus however many shieldstun frames shine has to retaliate or escape, which is quite a bit of room for a lot of characters. Granted, some people might feel pressured and keep up their shield regardless, but it's looking like this wouldn't work often...
:/