Jethro Pugh is definitely a player who was underrated.His Leadership,excellent ability to play the Run and Pass Rush along with his durability deserve respect.Coach Landry held Jethro in high regard as did Bob Lilly,who praised him for contributing to his success as a player.
Dave Edwards was another Player who was underrated.He locked down the Strong Side LB Spot with his Strength and Intelligence for 12 Years after Coach Landry flipped sides with Chuck Howley and was a solid,significant part of those Defenses.He also was given praise and respect from Landry and his Teammates.John Madden actually showed film of Dave Edwards on how to defend Tight Ends getting off the Line of Scrimmage to his Oakland Raiders LB's in the 1970's.
Cornell Green,George Andrie,John Niland,Larry Cole,Jim Jeffcoat,Mark Tuinei and Tony Tolbert were also underrated and important players IMHO.