Luigi's platform game overall is better, allowing him to keep his wavedash momentum when he phases through a platform and attack with any aerial. It sort of makes up for his awful air speed and is a very, very good tool for approaches. He can't really do this in melee. I can link you a video where it shows this. Although i really don't want to as it was about 10 months ago and i was awful then. I'll link it, whatever. Go to 5:42- 5:47
https://www.youtube.com/watch?v=iW87Dd2PoJE and notice how I go from the right platform to the left side of the stage. This is 3.0, but you can do this in 3.5 too. And you can go farther if you have a full waveland. All chars with a good wavedash can do this, but luigi's is the most effective. Also I played horribly that set (still won, but i'm much, much better now).
Luigi can also hold R or L while facing away from ledge and carry the wavedash movement with him, allowing you to go extremely far and close to the side blastzones and finish your opponent with a back air. It's very akin to a vududash except it's easier to do (although vududash isn't too hard) and the only aerial you can really use is bair (you can jump out of this after about half a second and bair)
Luigi is also able to wavescuttle, essentially an extremely fast and funny looking crawl. Basically you do a full wavedash forward, and then crawl backwards (crawling from right to left), allowing you to keep the wd momentum. It's kind of like a regular wavedash back but is a bit faster and allows you to avoid projectiles that the opponent might shoot (reflected fireballs from a nayru for example). You can also do smash attacks a bit faster this way.
Vududash is still possible
Luigi's down b is much better. You don't have to charge it, it's easier to rise with it (much easier) and it's a great onstage approac option. You can also rise with it on the ground and follow up with an aerial. 3.0's cyclone was the best as it had less ending lag, but this one is great, as well. I've gotten a lot of tornado to misfire combos hehe.
Misfire is a bit weird. One in every six green missile will give you a misfire (you can store the misfire as well). This makes recovery a lot better as you are free to choose when to use the misfire (although it takes away some randomness and surprise). It's also a great combo finisher when combined with your regular aerials. It's a great change but i hear a lot of complaints about it being too cheap. Thing is the hitbox is on luigi's neck, so any hitbox or projectile stops it dead on its tracks. It's also slower and does less knockback.
Fireballs are a lot better. They go a farther distance, are bigger, and have less endlag. You can sh fireball -> to waveland, which is an amazing tool for approaches (timing is strict however).
Platform cancel aerials are a bit harder to do as the timing is a lot faster. I also find ledgedasing in PM tougher than in melee for some reason, but it's easy once you get the hang of it.
Up B now has horizontal movement, so it's easier to use for recovery and escape punishment if you whiff. as you have a much better chance ledge cancelling it. Up b also seems easier to hit with now. Out of shield up b's are so nice, it also seems to go higher than melee's and has less landing lag.
Dtilt has been changed as it doesn't pop the opponent up anymore. but is a mr saturn type move that does more damage. It does 15% no matter what, has no knockback, and does considerable shield damage. It's also great for jab resets as you can rack up damage with it and finish with an up b once the opponent gets up. Great change.
Dash attack is also much better. It's not really unsafe on shield anymore as it does considerable shield damage, and it's no longer really unsafe on hit because the opponent is knocked back. It's great and a much needed change.
Side B can grab ledge but you have to be directly underneath the ledge.
Luigi in general is a lot better in my opinion. His platform game, punish game/combo game have drastically improved, and his approach options aren't so limited anymore. I do think that he still loses to a lot of matchups, but if he puts in the work, he can still do really well. I put him a solid mid tier right now comparatively to the rest of the cast.