His forward Smash KO power was really nerfed. Even Mario's FS kills earlier (if sweetspotted), although, to be fair, Mario's was hugely buffed. On the up side, Luigi's comes out quicker than ever and has better range (I think) making it one of the fastest FSes in the game, if I'm not mistaken, while still having good KO power.
If you angle Forward Smash downwards or upwards, its potency as a KO move increases.
Fsmash Upwards: Raises ability to KO at the top blastzone by about 10-15%.
Fsmash Neutral: Worst ability to KO, but slightly increases horizontal range of the attack.
Fsmash Downwards: Raises ability to KO at the side blastzones by about 10-15%.
Luigi is probably going to end up being Low Tier, but not bottom tier. In fact, I feel as though his potential is higher than Mario's/Dr. Mario's (whom I feel as though is bottom tier). Luigi has range problems, trouble KOing, trouble approaching, slow air speed, bad traction, among other problems. However, his strengths over Mario include better damage output, better zoning with fireballs, better jab, better techchases with Down B and Forward B, better aerials, probably one of the best Dthrows in the game for true combos and dealing damage (overall), better recovery, and being harder to approach than Mario.
And there's always Up B, which is one of the strongest KO moves in the game. Despite it being ridiculously unsafe if you miss. And you can combo into an aerial Up B after a Dthrow at certain percents.