Depends on the material...
Passion & Warfare is probably THE guitar album of all time (for a solo musician as opposed to a band collaboration).
His work on the "Crossroads" movie is outstanding, especially "Eugene's Trick Bag".
He stole the show on Whitesnake's "Slip of the Tongue" where "Sailing Ships" is one of the greatest songs ever.
His work on DLR's "Skyscraper" and "Eat 'em and Smile" is great -- check out "Hot Dog and a Shake".
So I don't really agree, but I will give you this - aside from Passion & Warfare, Steve Vai is better off playing in a band whether it be Dave Lee Roth, Whitesnake, or Sex & Religion.