The problems with Luigi is his Priority, random kill power, and touch of death combos.
Luigi fights like he is dual wielding swords. He has more range with his attacks than he should have, and a lot of his moves beats out other people's moves.
He also has moves that randomly kill. His down B shouldn't kill and his a air shouldn't kill. Seriously, why does his Nair kill? It is quite literally the only Nair that kills in the center of the stage. No one else has that. His Nair also comes out in less than 5 frames and can A. Catch anyone by surprise. Or B. Punish and kill some one who is comboing you. Why should they be in a disadvantage whenever they hit you? Traditionally it is the other way around.
With one jab, grab, or Fair Luigi can kill you at any percent. The right amount of fair, up tilts, jabs, and down throws he can drag your HP from 0% to over 100 in a matter of seconds where he can finish you off with a well placed Nair, Bair, down special, side special, or Smash attack of his choice. Or he can just drag you off stage instead and use his insane recovery abilities to make it back to the stage.
Seriously, fuck Luigi in this game.