Well, of course. He's gotten a few inconsistencies, like with that whole Castlevania bundle thing coming to switch with Simon's announcements, but he's been pretty on point when it comes to characters.
You could argue that characters like Chrom, Dark Samus, Isabelle, and Ken were predictable, but you can't quite exactly say the same for no one being cut, Daisy and Ridley getting announced, and Snake being back in as predictable. In terms of Smash Bros. content, he's extremely reliable. But he doesn't seem to know everything, and if he does, he leaves details out.
There's the possibility that he'll end up coming out as a fraud in the future, or he'll make some major slip-up on a character. But, aside from a real Nintendo employee, an employee of another company involved in Smash, or Sakurai, Vergeben is the most reliable source. Not perfect, but he's got a far better track record than any other leaker out there when it comes to smash.