Peppard's failure in "tough guy" roles.
I like George Peppard as an actor, but I think he was somewhat unsuccessful when he tried to join the ranks of actors like Lee Marvin and Clint Eastwood in movies like "Cannon for Cordoba" and other Western and Military themed movies in the late 60s and 70s.
Rather than seeming genuinely tough and dangerous like the other actors in this genre, Peppard's take on cool tough guy acting made him seem merely peevish and annoyed. He was too urbane to play a hardened gunslinger or soldier.
He should have concentrated on dramas and romances, or more sophisticated action heroes in the James Bond mold.