Mario's WoP with B-air? Never saw one before. U-air strings are awesome on Mario, but only works at low percentages.
Luigi himself can string aerials very well (Surprised because 2nd slowest air speed? Nah). Though, the execution isn't as braindead. I have seen Boss doing something nuts like D-throw -> Double chop -> Double U-air -> That does like... 46%? On Pit? Okay.
Furthermore, Luigi can actually compensate his slow air speed by using the ground to continue the chain, at a price of harder to pull off, but the reward are generally very... well, rewarding lol.
Luigi can also combo at high percentages with D-air spike bounce. Sure, people may go like 'hurr durr... you can just tech the D-air hurr durr', but I fail people doing that flawlessly, unless we are talking about bots, Even lvl 9 CPUs have some trouble teching Luigi's D-taunt continously (And this is the same lvl 9 bots that can PS three Mega Busters in a row). And when they bounce, you are basically in D-throw position where you can throw out any aerials, Cyclone, or even FJP. My favourite would be D-throw -> D-air spike -> Another D-air/N-air/B-air. Those does 25+% (Save for second D-air sourspot, which deals 24%) each, and works until something like 235%. That's something to note, especially since Luigi's finishers aren't as strong as it was in Brawl.
Mario himself can combo out of his D-throw at late percentages with SJP, but that doesn't do much damage or as safe as Luigi's D-air spike. Heck, Luigi can actually get 20% off two hits with his D-throw + B-air, and less the hits needed, the better (As long as it deals a lot of damage).