This thread should be on the Luigi board. You'll get more answers there if nothing else.
There are guaranteed setups for the up-B sweetspot (at least when ignoring SDI; I'm not really sure how much it changes things). If you don't already know, the aerial version is much weaker than the grounded version.
Honestly, it's only very recently that I've actively been trying to incorporate it, so I can't give a whole lot of detail here. However, I'm still confident that up-B is very powerful and underutilised, so I'll be looking for more uses.
Late nair can combo into it at a decent percentage range (fast-fallers like Falcon and Falco are more susceptible to it). I'm pretty sure that in at least some situations it's possible even if they DI to the side (I'll have to test it properly; I've never really been conscious of it); the strength of the nair seems to be much more significant than the opponent's DI. As an aside, this is one of the reasons why late nair is so great in general; it combos into so many things in so many situations. Up tilt and up aerial can set up for up-B as well (though up aerial is much more DI and percentage dependent).
You can combo from a rising late nair into aerial up-B to kill off the top at approximately 10-30% earlier than a strong nair will (you should just use strong nair instead if you can kill with it; it's lower risk if you miss); this is only really useful at high percentages near the top platform/blastline. You can also combo from a falling late nair into a grounded up-B, which kills really early (even in situations where any other kill move will barely even get the opponent offstage, if that).
It works as a punish on missed techs and read rolls/spotdodges; as a punish on moves like Rest, Counter, and Samus' up-B OoS; and as a punish for things like Marth's forward smash on shield and whiffed tether grabs. You can tech-chase with it like you would with a grab. Practically, if you have enough time to grab then you have enough time to up-B sweetspot (though obviously up-B won't work on shield). You can also use it out of a run or dash which extends its usefulness a lot.
When the opponent recovers onstage near the ledge you can waveland or regular stand from the ledge and up-B them; this is really easy/common/effective against Sheik in particular, but works on a wide range of characters (Marth, Falcon, Samus, and Luigi being some of the more extreme/notable examples).
If the opponent is spacing really deeply into your shield then you can up-B sweetspot out of shield, though this is rarely useful since good players will rarely be spacing like that.
Jab to up-B works against floaties; I've mostly seen it used on Samus and Marth. Unfortunately I haven't really incorporated this so I can't be much more specific. I'm almost certain that SDI will make this impossible.
Sometimes YOLO wavedash/dash/run up-B can work, though it can often be substituted for an up smash to similar effect but with less risk. You really shouldn't even consider going for it unless you have a read on your opponent and you can kill even if they DI. This is less risky if you can edge cancel; in this case if they avoid it by shielding then in many situations they won't be able to punish you beyond taking stage control (which is still a pretty big deal), and if they avoid it by moving out of the way then it's not as bad a situation as it would otherwise be.
Also, a gimmick: if you are against the tree on Pokémon stadium and up-B a shield then you will be safely pushed back to the other side of the tree, edge cancelling on the way.
