There's a lot of pretty well established theory about why customs improve balance... and also the actual experiences of those of us who do play with them. It's not like it's a thing no one has ever done; some of us are very familiar with the system. At this point the evidence strongly suggests customs improve balance and the burden of proof would be on the other side; I haven't seen any solid piece of evidence that customs are any kind of a balance problem as opposed to a balance boon.
Pit vs Dark Pit is too boring, sorry. His hilarious "Where's your goddess now?" win quote is near the top of his list of positives, but he's at least so close to identical that his downsides don't matter much...
My issue with Luigi is that, while his frame data and speed to power ratio are the best in the game which is no small thing, his mobility is fairly poor overall especially in terms of air speed and that makes his neutral pretty hard in some match-ups. He has super stubby limbs and even less average range than Mario to go with this. I'm legitimately unsure how Luigi approaches Villager as an especially notable point of concern. Luigi's recovery is also really gimpable even moreso than Mario's even if the distance can be good if you have superhuman mash abilities; that also concerns me. His traction also makes him miss out on some punishes and makes some neutral stuff hard on him (like run up shield is only a viable approach for him if you powershield); it's really not a good thing. He's a good character but he's flawed whereas Mario is weaker in his core competencies but also significantly less flawed. While I'm not inherently wedded to well-rounded-itis, my rough estimation is that Mario comes out ahead in this. The high tiers are also crowded and Luigi is kinda difficult to place; I probably put him on the low end of reasonable but some of that was having optimism for other characters and not just pessimism for Luigi. I do think the balance in this game is pretty good so a third tier character is still a pretty good character.
I think Lucario's neutral is being understated by some of the more extreme claims. Is it as good as what Diddy, Sonic, Rosalina, Sheik, etc. have? No. Is it legitimately bad? No, not really. Lucario has pretty good reach on a lot of moves, a good projectile, a counter, a command grab, decent movement overall (middling ground speed, floaty air physics but a large double jump to compensate and a dair that plays with his fall speed), and middling frame data lacking super fast options but also not burdened by overwhelmingly slow options (very similar to Rosalina w/o Luma on the frame data front really). I think Lucario's neutral game is about average all things considered whereas his pay-offs are huge. If his neutral game were actually horrible I'd probably look at him differently, but I don't think it really is.
What I meant by his reads not having to be in a row was that they don't have to be immediately subsequent. Like let's say Sheik wants to kill really early by making good reads. She can do this but she has to just chain them to keep hitting you and dragging you off-stage for a gimp. Her opponent must make many mistakes/get predicted many times in a very short time frame with a reset to neutral ending the party. Lucario's three reads for a kill at high aura don't have this limitation. He can get one good read, allow a neutral reset, you can run around poking at each other for 10 seconds, he can get his second, 10 more seconds of neutral, and then he gets his third which should be Force Palm grab and the kill. That's a big deal. Obviously if you get a read on Lucario and end his stock the party stops, but my point was that Lucario's ability to capitalize in an escalating way on getting reads in high aura is about not getting read himself and not about avoiding neutral reset like for so many other characters. I value this highly since it puts the burden of avoiding a big loss on his opponent's ability to take a stock not escape a situation; clearly this is controversial. I don't really want to just argue a lot about Lucario; he's only one character and isn't even seeing a ton of play right now. I just really believe that the aura mechanics are very strong on top of an otherwise pretty decent character.