Snake
Snake struggles with Sheik the same way others do, in the sense that she's really good at applying pressure while holding space and there's not much he can do about it.
If a character can threaten him without getting Grabbed or clipped by UpB OOS, then he's in a pickle. With good aerial/ftilt/dtilt/jab, and a Grab to match his, she can just kind of 'be near' and he has to do something about it or he'll be on the receiving end of some kind of mix-up or needing to defend himself or lose stage, etc.
Zelda actually works well against Snake too, and if it weren't for his vertical kills being so centralizing to him then he'd be in major trouble. It's the same way Ike/Ivy/Puff can mess him up. He doesn't have the distance-closing to turn a rising aerial from them into an offense, and can't evade a following/falling aerial without losing stage and not being able to come back and punish it, or possibly being totally poorly spaced if he runs in to try a counter-offensive. Other characters (everyone from Mario to Fox) can at least work around things like rising Fairs in threatening ways, even if they shield it. Snake can't do crap unless he hard-reads and messes up spacing.
It's kind of why a standing Ike/Ivy/Puff/etc will be more of a threat to Snake than someone like a DDing Marth or DJCing Ness or whatever, who uses movement to control that space. Snake doesn't have what's needed to 'gain' stage if the opponent doesn't go anywhere and just winds-up hit after hit, or sits at that Sheik/Ike/etc range where if Snake does an action that doesn't hit, he gets smacked, and if he doesn't do an action, he's waiting on an unfavorable RPS situation.
You can even see this in really old tourneys, where Zero's Fox would macro-space with lasers + nair>shine>nair and go back to lasers. Don't need to play in middle-ranges or try to do mix-ups with DD speed AT ALL because Snake folds to simply being at a space that's threatening while the opponent doesn't even try to make openings. Patience and junk, check for that stuff next time you watch Snake's. It's pretty easy to see.
Puff
It's been a while, but I think it's everything but Jab and projectiles (and Grab) by Samus on Puffs shield are 100% guaranteed Rests. Unless light-shielding. And I think it's like 1 frame (so TAS needed) if it's on tippered Ftilt and Zair. Still hilarious though.
Puff is polarizing and still works great.
I won't make an argument for her... but I'll say that there's something worth considering that doesn't seem to get enough consideration when it comes to her.
Most of the PM cast go things that are useful for clipping opponents out of DD/speed movement, and comboing, and edgeguarding, and that's all great against all of the Melee-tops. But this stuff doesn't really apply to Puff. She's probably one of the few who can't even get close to Dthrow>Aerial killed by Mario and the like, doesn't get edgeguarded at all (she doesn't die early, because she has to HIT the blastzone, which no other character really 'has' to do to die), and she doesn't rely on grounded safe DD game where things like safe mix-ups on shields and covering dash/WD distance is really a factor to get hits on her.
What's needed for Puff to have trouble is either great controlling walls that can deal with her aerial disjoint (Marth Fairs or Falco Bair/Laser combinations), and an airborne kill-move that can connect on her. Meaning the aerial mobility to land it, and likely something else to lead into it (think CF/spacie Bairs/TL/etc, there's a reason peeps like Pika can't deal with Puff without good aerial kills in Melee, and Peach can't without the range and mobility to connect her needed hits)
And yeah... not too many characters actually have that in droves. They're all CAPABLE against Puff to some degree (Ness Uairs and Lucario Nairs and Snake Bairs hurt) but to land those things on a spaceship is hard af when that spaceship is across the stage and hitting you.
Puff has it no worse than other Melee-Tops when it comes to the transition to PM. That's all a myth. But she's defs polarizing (duh, she's a spaceship in a smash game) so isn't gonna be crushing tournaments solo or easily anytime ever.
Kirby
This character would probably be seen in a different light if we had super campy players like Chu just DD grabbing for days still.
Likely what makes this character 'suck in the eyes of smashers' so much is how there's no solid combo chains that can 0-death the way almost everyone else can, can't grab > finish the way almost everyone can, can't control space with moves and speed to bait and punish the way others do, etc.
But Kirby is by no means dysfunctional.
I won't make an argument for Kirby being 'good' but Kirby has a place in this game on a top-end competitive level for sure.
My dream of having 64 Fair (which would fit perfectly with PM Kirby) is gone forever, but Kirby is still solid, and a blast to play. When grounded-safe-stupid play becomes more of a thing, and players are as smart about holding position as plups sheik against Fox's, then Kirby will rise. Until then, in any unrefined meta-game, Kirby is gonna wallow in trash tier.
Can't flail = gg character at the current state of the game. Probably gonna be a while.