I agree to some extent with your notion on what an under-rated director is considered to be. In response to this thought, I must say that Buñuel has not received the recognition and/or acknowledgement that he deserves by critics, viewers, or peer filmmakers during his film making career, in an overall sense.
This having been stated, I think that Buñuel has received much lesser acknowledgement for his prowess in the film making industry in comparison to quite a large number of directors whom unrighteously gained more recognition than "the Master", Buñuel.