I'm still not sure who this match-up favors as I would need to play it more, but I think regardless of whose favor it is in, it's pretty negligible.
There are things Sonic has against ROB that he can abuse the hell out of, but the same can be said about ROB against Sonic.
If you let Sonic get in and have his way with you, you're going to get utterly mopped. Sonic's can combo ROB very easily due to his giant hurtbox and difficulty protecting himself from below. Uair strings are incredibly effective, and comboing into Uthrow>Spring>Uair or just Uair follow-ups/KOs in general are quite easy. When Sonic gets his momentum, it's some of the most fun I have playing this game.
On the flip side, however, when ROB zones the way he's supposed to, it's some of the least fun I have playing this game. My sets with GwJ are incredibly varied. I either bop him, he bops me, or it's janky overall.
Sonic has a pretty safe time edgeguarding ROB, but gimping is heavily reliant on stage spiking which can be teched. Otherwise, we'll just take our free damage and call it a day (Dropping springs, etc.) Even then though, ROB isn't completely helpless. The hardest thing for ROB trying to recover is that he can't attack WHILE using fuel, so Sonic can stuff that... ROB has to play very preemptively here, but I feel like that's how ROB has to play in general. Sonic will rely mostly on Uair, Bthrow, and Bair kills in this match-up I'd say, as smash attacks against ROB are so much more of a commitment that he would have an easier time dealing with.
The reason why ROB can do so well against Sonic though, is that Sonic's approaches are really awful. Our best approach in this match-up (and almost all match-ups) is shield... Yup. Against characters that don't have great zoning options, this isn't too bad for Sonic because of his plethora of mix-up tools... But against ROB, they can be rendered pretty useless if you just stay away from us. If we don't apply the pressure quickly, then it's a complete headache. ROB's gyros and lasers are effective, but outside of that, his tilts have good reach and his Nair surrounds him in hitboxes which really effectively cover Spin Dash/Spin Charge Rolls and Spin Dash/Spin Charge Jumps. It covers all our angles if we commit to a spin too much.
ROB's Uair also saves him imo. The fact that it's a kill move now is so good for him because Sonic's landing options are his other weakness outside of bad approaches. Catching/baiting/punishing our landing options is really easy for ROB since he can pressure you by putting gyros on ledges, pressuring from afar with lasers, and all that annoying stuff, so forcing us into bad positions is really easy and ROB can capitalize hard on that. Our landing options are far and few between, and if Sonic tries to land with Dair one too many times you can just Uair/Usmash us for a KO.
So basically, ROBs goal is to shut down Sonic before he can get too crazy. Sonic's approaches are only intimidating if you don't know how he works or what mix-up we're going for, but otherwise, are all easily outprioritized and really easy to camp out. However, if you make a bad call or leave yourself open, Sonic will completely hound you.