To answer your question I think Fox is going to be another tough matchup for Ness(perhaps why you're considering him as a secondary?). Shine is bad news for Ness if he reads a pk fire as it confirms combos and allows him to not care about thunder if he's offstage. Fox in general with his recovery mix ups and shine will be difficult for Ness to edgeguard. Because a Fox still has a fantastic combo game and good kill power on usmash, uair, and bair he has no issues taking stocks from Ness.
I think Pikachu is another matchup that's gonna be tough for Ness. His edge guarding is excellent and his combos and frame data mean that the Ness will be offstage a fair amount, a place where Ness you as the Ness don't really want to be. Quick attack make Pikachu borderline impossible to edgeguard and his speed, small hit box, and other tools give him the ability to win neutral. Pika also has a disgusting advantage state.
Some Ness vs Fox sets for your entertainment
BestNess vs Larry Lurr
https://ssbworld.com/watch/?v=rV4SY1FzPNE
Awestin vs JaySon
https://ssbworld.com/watch/?v=nHWYdAXHC54
What are the takeaways?
Well, that Ness has very sound counterplay to the situations you present.
- Notice how Shine is not the issue for PK Fire usage - it is just as effective (and more sensible for the Fox player) for Fox to simply jump and punish on reaction.
- The Fox players do not attempt to Shine PKT when Ness is recovering. BestNess simply doesn't give Larry the option most of the time, but in general you can see that Fox would rather stay firmly on stage. There's a good reason for this, and Envoy of Chaos has kinda touched on it: it's quite often not worth the risk for Fox to attempt an edgeguard.
- At 3:47 of Awestin vs JaySon, Fox goes for a Shine attempt to prevent the PKT edgeguard but Awestin simply uses PKT to force JaySon to stay in Shine, trapping him into a low recovery that Awestin nearly spikes. For matchup theorycraft purposes, you can hopefully see how this kind of thing should be a free stock for Ness. In general, no matter how Fox recovers, you can hopefully see that Ness can handle this easily. BestNess may have dropped some edgeguards which might alarm you, but that's nothing to do with Ness: the character clearly has everything he needs to handle Fox's recoveries consistently and effectively, regardless of how Fox chooses to recover at any given time.
- Fullhop nair (and other aerial approaches) is something we see a lot from both JaySon and Larry to try to get a good angle to hit Ness from, and is certainly one of Fox's more practical ways of getting something going in the mu, but as we can see Ness is not overwhelmed and in fact has responses to balance the risk-reward. What is key here is that Fox has trouble dealing with Ness' hitboxes and strong out of shield options. Fox doesn't really pressure Ness for free like he can do a lot of characters so that's something really nice going for Ness. Makes it harder for Fox to get Ness offstage just by using his frame data and what have you - Ness' own frame data and hitboxes are good enough to keep par.
- We can also see that Fox's combo game on Ness isn't too terrifying: observe how the Ness players use Nair a lot to challenge Fox's aggressive combo/string tools. Probably the clearest example of how this works can be seen at 6 minutes into Awestin vs JaySon or 2:30 in BestNess vs Larry where they use nair to break out of low % nair strings and this basic interaction works in most situations where Fox might threaten Ness. Naturally this isn't infallbile - Larry shows us how effective it can be for Fox to just wait for Ness to put out an attack to challenge Fox and punish readily - but it is an important aspect of the matchup: Fox can't keep risking trading with nair (he's too light and vulnerable) and Ness can leverage this to his advantage to minimise the damage he takes when he loses neutral.
Hope this was of use to you and some food for thought? Hope to see you around to talk about Ness some more anyway!
I'd love to do the same thing for Pikachu, but this post is big enough tbh and I don't really have the time. Maybe you can check out some Ness-Pikachu videos and compare your points, as well as think of other important parts of that matchup you didn't bring up?
Ness definitely feels like a high tier in this game. Who would you say gives Ness trouble?
I know you didn't ask me lol but while I'm here. His tougher matchups are what you'd expect them to be. Swords mainly, or at least the good swords. Characters that can safely control a lot of air space with hitboxes Ness struggles to challenge or otherwise work around. Finding vulnerabilities on fast sword characters is pretty hard for Ness and he can struggle to pose threats in neutral, mostly resorting to whiff punishing an aerial when possible, and of course getting aerials in when he can and making the absolute most out of any hit he lands on a shield (or indeed the character) with solid reads on what they'll do next and whatnot. You can't waste any opportunity like that, because god knows when you'll get another.
That said, when it comes to advantage, PKT is very good against all the sword characters to juggle, edgeguard, gimp and set up, and give Ness the ability to carry his neutral wins against them very far in this game. If he didn't have that crazy potential there these matchups would really suck.
Aside from that, his matchups seem pretty alright to me honestly. Even Belmont and that. We'll see what happens, there's a lot of development to undergo for now before we can really be confident on anything.