I don't think 6.Qb6 is a very dangerous line for black.  What are your results from "closer scrutiny"?   

I don't think the 4.Qxd4 will refute the 2...d6 Sicilian  (which I guess the title of the post is suggesting?!).  In most "normal looking" openings, there is rarely a simple answer to the state of an opening, so it's not often that a whole opening will become unplayable due to problems in a single variation.  Of course, some people like to exaggerate and be overdramatic...(like van der Weide (on the Benko) and van der Sterren (on the Breyer) in New in Chess if memory serves )