Luigi sucks on RC. The stage scrolls faster than Luigi's aerial moving speed. In the air, if Luigi goes against the scrolling, he is at best stationary.
The scrolling only helps Luigi by making his dash attack unDI-able. That's a reeeeeeaaaally small counterpart.
Besides, the stage kinda prevents juggling, because you're never 'forced' to land anywhere (especially on the rising part). That's good for diddy, and bad for Luigi.
This is false. Diddy can get followups from bananas if you glidetoss to where Luigi is and he's near enough. Dsmash, ftilt, another banana, and probably some other stuff that doesn't really matter all work. It's true that you don't always get followups on him that would work on other chars, but there still are followups so what you said (and I read this often already from many people) isn't really true.
Yup, it happened to me a couple times, but if you keep Luigi away like you should be doing, you won't really be able to land ftilts and smashes.
When Luigi reaches death percents, that's actually an advantage for him : he can just choose to land far enough and with a backwards momentum to make sure he doesn't get smashed out of the bananas he may take in the face.
As for Luigi approaching with a banana... I don't really see it being easy/that rewarding for Luigi. If Diddy puts the other banana in front of him Luigi can do NOTHING but wait, or roll through it right into Diddys arms (or jump and get fair'd/Diddy runs away - if Luigi jumps Diddy can always retreat since he's falling so slowly and is moving really slow horizontally too).
I never said that it was easy, I said it went from 'impossible' to 'possible'. It's giving a fast projectile to a character whose main weakness is his slowness.
However, I don't see how a banana is NOT rewarding for Luigi. If he hits you with it, he can land any of his smashes so you'll eat at least 20%, be in the air/get killed... and let him pick up the second banana so he can maintain his newly gained momentum. You can't say the same for diddy, that's why I said bananas are more rewarding for luigi than diddy in this matchup.
Other than that - if Diddy doesn't put the banana in front of him all Luigi can do is glidetoss (which is still pretty laggy). I don't really see it too treatening, because of it's slowness and yeah... if you just shield what's gonna happen? Just don't get hit so you won't get upB'd. It's not that difficult to avoid getting hit and tripped (be in the air, jump on platforms... just get the other banana and then approach with shielding can also work since Luigi wants to get rid of the banana and you don't care if you have one).
Luigi's back glidetoss is indeed long and laggy, and that's why you won't see it used offensively really often. It's mainly used to punish any zoning mistake from far away.
...Like a diddy camping on a platform. I wouldn't do that if I were you. Rule n°1 : stay under, or on level with Luigi.
Why Delfino? I like Delfino. :] I'd also ban Halberd probably (although Diddy still wins there, but upB / other stuff definitely is even more scary there).
Luigi sucks on the ledge, and the stage gives him other recovery routes : he can now go through it from under, or plank until the platform lands. Either way, giving him more options kinda negates this weakness.
There are also a lot of slopes, and slanted grounds offer a protection against bananas.
I usually CP this as Luigi if brinstar is illegal and halberd is banned. Well either this, or Yoshi's island (hitting maskasses refresh my bair and nair so I have additionnal killmoves).