As for my personal opinion on the Sonic content...
It's complicated.
Twenty tracks and two stages are fine, but the second stage is an underwhelming choice (which certainly looks like it was chosen with a promotional angle) and the amount of tracks introduced in 4 / Ultimate is a bit on the small side. Not as ludicrous as the 5 years FF spent with the "2 songs" meme, but still small enough to raise an eye. There's a legitimate point to be made that Castlevania and SNK have set the bar for "generous" music licenses and perhaps - if I may be so bold - "spoiled" people, although there are a lot of follow up questions to that as a result.
As for Sonic's moveset, some of my issues with it are also applicable to fellow Brawl newcomer Toon Link - it feels like they're sticking a bit too closely to how Sonic was first implemented, instead of taking the time to try an implement a new B move or whatever. TBF, he did get a new dash attack and adjustments to how the moveset functions overall. Others come from personal frustration with how the character plays (although that might be mostly the online really - we'll have to see when offline tournaments come back in full force). I've warmed up a bit to the moveset since it can certainly be used aggressively, but there are still a couple changes I'd like to see.
Sonic's moveset has never been an issue to me. It reflects his abilities in the games just fine: I think the problem people take with it is probably more related to him being annoying to fight on quickplay lag, and yeah, I get that. However, that's not really a fault of the moveset, it's more of a fault of the online being so poorly made that it gives certain character archetypes like Sonic a perceived advantage when there's lag.
That's certainly the main part of it.
Sonic is buffed on Wi-Fi because the online lag / netcode makes a lot of Sonic's moves much more difficult to react to. Competitive people were forced to migrate online due to the lockdown and everything: it's the big reason why there was this wave of dislike heaped at the playstyle a 6 or so months ago (similar to the dislike Min Min's currently facing amongst Japan's competitive scene, although the two examples are a little different).
There are some aspects of the moveset I'd like to see changed, as said before. Two examples:
* If possible, merge Spin Dash / Spin Charge, tweak the merged move so he can access either (but making it a bit less safe) and give him Lightspeed Dash or Boost as a new Side B. It reflects my personal preference towards aggresive play, but giving him one of these would be very interesting potentially commital options. Spin Dash / Spin Charge being two separate moves seems a bit superflous or even a little confusing, even if my opinion on those two have calmed down a bit.
Or:
* Some changes to his aerials. I've seen Sonic Eagle from Sonic Battle used as a F-Air (in Project M for example) and it's a great example of what I mean. Related to that, give him a bit more flair pls.
To be clear, I don't want to neuter the hit-and-run style completely, just tone its presence down a tad and introduce aggresive elements so Sonic can play both more consistently.