There's a big difference between "waveshine sheik literally across the stage into downsmash into grab the edge, ledgehop invincible bair to regrab the edge to force an up b, to stand up to more free damage" and "waveshine -> single grab/upsmash". I don't think the former is reliable because of Smash DI on the shine.
One grab on Sheik will usually lead to one aerial and not a whole lot more. Ditto an upsmash if DIed away.
Samus has an unparalleled defense, pretty good zoning, and a mediocre but passable punishment game, which makes her like a bad version of Jigglypuff. Which is still decent. She certainly does better in the space animal matchups than characters like Falcon and Ganon. Compared to the rest of the mid tier cast, holding your own against both Falco and Fox is pretty good.
Amazing by what standards? He's a decent character, but just going even with Fox/Falco while struggling with Jiggs, getting dismembered by Sheik, and randomly having tough matchups with mediocre characters like Falcon and Ganon isn't "amazing". The Marth representation is more or less about where it should be right now.
As for sheik: read what KK said. It's possible to follow SDI, if they DI upsmash away and off stage they can get edgeguarded to hell. You can continuously waveshine until you notice SDI, THEN upsmash, which could be anywhere from 1-5 shines on FD. As he said, there's also a lot of 50/50 stuff. Maybe there's a situation where the only way sheik can get out is by nairing, so the fox just DD out the way and continues comboing.
As for what you said about samus... it's vague. Unparalleled defense? What about it is so unparalleled. There are so many specific things about the matchup that a blanket statement like that just glazes over. Just because she does better than others doesn't mean it's even, it just means she gets ***** less. A worse version of jiggs? Well fox already has the advantage vs jiggs, and falco vs jiggs is pretty **** close/debatable. If it's worse than jiggs, I'm pretty sure that guarantees that spacies>samus.
Amazing by what standards? In your analysis, maybe a bit less exaggeration needs to be done. He consistently goes even with debatably two of the best characters in the game (fox and falco).
He has the advantage vs jiggs... struggling with jiggs? He has grab autocombos, more range than her, similarly disjointed aerials... This matchup has been thoroughly discussed and I'm aware jiggs has a lot of things to fight marth with; marth might find a hard time with it, but it's certainly not to his disadvantage.
Dismembered by sheik? It's hardly dismembered. Marth's grab combos are more devestating in terms of how much damage they do than sheik's are. Edgeguarding is about equally stupid for both... The only sheik really has over marth is that her comboing is easier, and more consistent at higher damages. This doesn't change the fact that, as I said, marth's combo game is far more rewarding than sheiks. I have yet to see a valid argument for why sheik "dismembers" marth other than ease of the matchup. Just because its easier for sheik doesn't mean much. Look at fox vs jiggs. Upthrow/uptilt rest is far easier than anything fox has, but that hardly means jiggs dismembers fox.
Tough matchups against ganon and falcon? Marth has gimping and comboing galore on both of these characters. Ganon is too slow to take advantage of marth easily, and his amazing range that helps him in another matchups doesn't mean as much anymore when you're fighting marth because of marth's own range. Marth's edgeguard game is better than ganons, his combo game is better, he can sit there dash dance camping and just waiting for openings, and he matches ganons range.
Matchup against falcon is just marth players *****ing. Marth can CC grab any nair approach, and outrange any other aerial easily. If marth gets a grab at around 30%+, it's free uptilt and u-air comboing for as much damage as any of falconss combos do on marth. He can also fthrow/dthrow tech chase effectively on falcon because of his slow tech roll. His edgeguarding is far superior to falcon's as well. Normally falcon has to rely on DD camping, but marth has just as good of a dash dance, but a ridiculous grab range to go with it. The only thing falcon truly has going for him is a good combo game. Other than that, he loses to marth more or less in every single aspect of the matchup. Once again, the ease of a matchup does not reflect the matchup itself. Just beacuse its easier for falcon to do his combos, doesn't mean that marth's combos aren't just as effective. I mean, you said that marth goes even with falco (which I agree with). You said this despite the fact that marth has WAY easier combos, gimping, AND edgeguarding than falco does. Falco can do all those things just as well, but not necessarily as easily.
That said... Marth can go even with/beat the top three characters (fox, falco, jiggs).
Marth, at worst, has a 60/40 matchup with sheik (I personally believe it's either 55:45 or 50:50).
Marth at worst goes even with falcon, and obviously beats ganon.
Marth beats peach and ICs, and completely annihilates samus.
Honestly, marth's only real disadvantaged matchup is sheik. Even a worst case scenario for him, he still only loses to falcon and sheik, and just barely to both of them; in return, he goes, at worst, even with the top three characters in the game, while everyone else has problems with them. Definitely sounds amazing to me.