what are you talking about? they buffed his dsmash and his dair is now basically melee falco's
The irony that Luigi's Down Smash was always a good move, but nobody really bothered to use it. It's a frame 6, 14-5% move you can act out at frame 38. Luigi's Down Smash has the same recovery as Meta Knight's and lower recovery than the Pits, Little Mac, Rosalina, Peach, and Zelda who arguably have the best Down Smashes in terms of utility. They might not kill well except for Little Mac's, but they are pretty much the safest cover around and get off me moves on the ground. Except for Little Mac (frame 10), none of them are above frame 7 and Meta Knight's is frame 4. Ryu and ROB would count too if Ryu hit behind him, but it's based off of his cr. HK? which doesn't hit behind him because most 2D fighters don't have a lot of moves, especially normals, hitting both sides and ROB's kind of iffy at times. Also, Sheik's Down Smash is pitiful...
Also, hell no. Nothing is close to the brokenness that is Melee Falco or even Brawl Falco's Dair. Melee Falco's Dair wasn't even considered a spike due to programming error and who has a frame 5 spike with about 15 active frames, 5-24, in this game? Nobody. Brawl Falco's only spiked at frame 5-7, but still had 14 active frames for the late hit, 8-21. That's a total of 16 active frames. The closest Luigi's got is landing lag which is 20 to Melee Falco's 18 and Brawl Falco's 19.
Luigi's Dair spikes at frame 10 and has a late hit at frames 11-14. Only 5 total active frames with only 1 active frame for a spike. Luigi's Dair has more in line with the Pits except Luigi has a more reliable way to set his up while the Pits are more likely to be able to use their Dairs deeper off stage without major repercussions.
Luigi's Dair is only good because outside of Fox, Peach, (Dr.) Mario?, Lucario, the Pits, and maybe Kirby's, everyone's Dair plain sucks as spikes, aerials, or even just moves. Most Dairs are probably above - random number here - frame 14 contrast that to most every other aerial probably being below frame 10, especially with Nair. Zelda's Dair would have had an honorary mention if her other aerials weren't so situational which sucks since Zelda has a Dair that will spike no matter what - difference is the clean hit is a clean spike and the late hit is a soft spike. You're going get spiked or gimped and will likely die against her, but unfortunately... Zelda's bane is her existence.
In general, I think Dairs are pretty much the most useless moves for most characters. Most often than not, Dairs are designed to be situational moves, spikes. When a spike like Melee or Brawl Falco and Marth come up, those really shine because everything else sucks. When Dairs aren't highly situational like Fox, Lucario, and Peach's show up, it's made even worse for everyone else. You could for instance, replace Falco, Palutena, or even Luigi with his good relative to everyone's Dair with a
taco kick - don't question the name - and they would do so much better with a cross-up aerial than a spike. You do know how stupid it would be to have a low-angled kick that hits behind you, right? The fact Falco, Palutena, and Luigi have the most dangerous "back moves" would make it even stupider if they could jump over and make you guess which of their, basically, Bairs they will use.