I practice against a Luigi a lot, and I think that combing him is hard, but not as bad as people think. Comboing him just requires a lot of baits. You can do U-throw or D-throw into things, and a lot of the time its pretty guaranteed that you'll get something from it. Uair just space at the tippy-toes so that it doesn't trade as much, but if you're afraid of that then just SH or FJ next to luigi and do a late aerial after baiting out a Nair.
Watch out for Luigi's CC game, edge-guard game, and his tech-chase game. all three are very deadly to Falcon.
-He can CC most things very well up to 40% or 50% into grab, Dsmash, or something else. (He can even CC knee up to pretty high percents!! He will never punish you for a CC stomp though, I'm pretty sure you can grab if he's in stun.)
-Whenever you get hit onto a platform, either tech in place or tech roll in. Most good luigi's will be able to cover tech rolls easily since Luigi can just float in the air waiting, so if you tech outward you'll get Daired/Faired off stage and that's really bad.
-I recommend recovering high and fading in or out of the stage. If he hits you again, just DI up and keep recovering high. If he gets you below the ledge, almost nothing can save you from getting Faired/Daired, so avoid falling beneath the stage if you can.
To edge-guard Luigi's Up-B, CC at the ledge and D-tilt after Luigi takes a coin from you. D-tilt to knee or other punish is usually pretty given.
To edge-guard Luigi coming back to the stage from far off, just go out and hit him when he is in the lag of his green missile.
DI Luigi's throws, U-smash, and U-tilt behind him to avoid punishes better.