I don't think they've ever felt the obligation to fully illuminate the character's functionality with their reveal trailer. The point is just to make them look cool.
Take Steve or Kazuya. Complicated characters, certainly not entirely explained at their first showing. The intrigue of how their moveset works isn't a downside. At E3 they even basically teased Kazuya being elaborate and then were like "tune in later to find out wtf is going on!"
You're not giving people much credit for being able to tell existing movesets from new ones, which is a pretty rudimentary ability for anyone halfway familiar with the series.
True, it is most important to make the Character seem really Cool, but Here's the thing
It's all about the Surface with the Trailers, and unlike the other DLC Characters we have in Smash Ultimate, Shadow has a Really Similar body Type and look to Sonic, and I also imagine he's gonna share certain abilities like Homing Attack and Spindash (though I Imagine they wouldn't be Specials in Shadow's Case).
We have characters like Terry being called Hat Ken, Hero being Link Clone, or Kazuya just being another Shoto (despite him not being a Shoto), and none of those characters have such a close Resemblance to Another Character like Shadow does with Sonic, so it's gonna be even worse for Shadow in that case, because unlike the other Characters, You can much more easily tell the contrast between those characters and characters already on the Roster. If you just look at Steve, you already know he's Completely different from the other characters, and Same with characters like Min Min, Banjo & Kazooie. You don't really get that with Shadow if you aren't familiar with the Series, which while I'm sure Lots of people know of Shadow, they might not be familiar with his Actual character and abilities, and on the Surface, he is just Black, Evil Sonic.
I do Imagine Shadow would be one of the Flashiest characters in the game if he was added, but I think alot of people are still gonna just assume he's just Black Sonic, even if they see the trailer. Take FighterZ as an example, Despite the Goku's having different movesets, Alot of people still just see them as Another Goku, and with that being a factor, you gonna have to do more to sell the character
Alot of people would be able to Tell the difference between Sonic and Shadow just based of the Trailer, but alot of other people are not as well-in tuned with Smash to be able to even tell certain movesets apart, and Nintendo/Sakurai need to sell the character to these people too.
That's what I'm saying. Having the Demonstration just makes sure that everyone is on the same Page about Shadow being his own unique character, and not some sort of Echo fighter, because even if the Trailer does showcase crazy abilities you won't see Sonic do, Many people might just focus on the surface and just assume he is an Echo, so you gonna need to dispel that. yes