Let’s go over this in detail.
Sonic has quite a few things going for him. His speed, as you mentioned, is his most useful attribute which, in combination with his aerial mobility and spinshotting, can make Sonic extremely difficult to predict, much less react to. He’s got the combos, the tech chasing, and the gimps- all of which easily seem good against a character like Fox, but Fox has answers for all of Sonic’s approaches. Let’s look at two of the most even MUs between Fox and other characters from Melee to make this easier: Marth and Falco.
The attribute that these characters have that result in success in this MU, I believe, is their ability to keep the Fox off of the ground (and of course the other strong points like edgeguarding and grab game). Both Falco and Marth can tech chase Fox perfectly well, but doing so is only necessary when Fox hits the ground. Either of the aforementioned characters can not only stay reasonably safe from Fox, but can pick him up and not let him go. Sheik and Falcon may be able to 0-death Fox, but doing so is dependent on reads and chases. Falco and Marth can both kill Fox from anywhere on the stage with minimal open-ended situations. Sheik has to tech chase for 50%, Falcon has to hope his reads are accurate, and Sonic has to be as tricky as possible because he lacks in the ability to definitively combo Fox assuming that the Fox is aware of Sonic’s trickery and can DI and counter when necessary.
To be clear, I’m not suggesting that Falco and Marth can combo Fox and Sonic can’t, I’m saying that while Sonic can combo Fox, his combos are more dependent on tricking the Fox or the Fox making otherwise bad decisions. But that’s not all that sucks for Sonic.
As I stated in another post, Fox has heavy priority over Sonic. Sonic doesn’t have priority, he has speed and adequate hitstun. Barely any of Sonic’s moves can, from a practical standpoint, out prioritize a Fox who can space well and kick things with a few shines here and there. To make matters worse, and trust me on this one, combos for Sonic against fast fallers are few and far between. You’ll spend all your time running into surprise attacks and trying to tech chase and gimp, and all the while your opponent is learning and adapting. Uthrow can be DI’d, so far as I know, to the point where the spacie will hit the ground before Sonic can either regrab or tilt. A spinshot might work after uthrow, but I digress. Sonic’s utilt won’t work definitively until higher percentages, he has no vertical priority, HA gimps don’t work on Foxes who mix up their recovery, and his horizontal combos depend entirely on the Fox’s DI, teching, and the prediction thereof. But don’t worry, it gets worse.
Not only are Sonic’s offensive options unfavorable in this MU, but his defensive options are absolute **** against Fox. One prediction and you get your ass comboed with nowhere to go. If Sonic hits the ground post-knockback or stops moving ever, he’s probably going to get hurt or even die. Sometimes you can’t DI far enough because Fox is just going to catch up to you and kick you through the ceiling or nair you again. You’re going to get baired out of your spins and nairs and he’s going to grab you and have his way with you. He doesn’t even have to edgegaurd you. He’s got dem reliable ceiling kills that make edgegaurding Sonic nearly irrelevant, save for the fact that high-priority bairs and shines destroy Sonic’s once OP recovery. Oh, and all those things that Sonic is good at? Tech chasing? Gimps? Shield Pressure? Fox has them as well, and is better at a lot of them. Don’t let’s forget about Sonic’s snail-paced rolls and techs.
This MU is doable for Sonic in a Sheik/Falcon sort of way. You can win, yes, but you’re more likely to lose provided the Fox has Sonic experience. Put simply, you have to be very smart and super patient. The Fox player has control almost always.