While not radically different in the first two games, Luigi really stands out from Mario now.
Especially with his Luigi's Mansion stuff influencing his combat moves. He's not really an "echo fighter" like the days of old. Though this has been no different since like Brawl.
His Final Smash the Poltergust G-00 is the same as Smash 4. He uses the magic vacuum to create some havoc, though I thought it was weak and not very useful in comparison to other Final Smashes.
When Kirby takes Luigi's powers, he has the green fireballs, like it has been since the N64 days. But at least Kirby still looks cute in his Luigi hat.
Online against foes, I did okay with Luigi. While I had been outclassed a few times, other times, the Green Plumber has held his own weight.