I always thought the Frank Burns character was too over-the-top even for the comic nature of the show. Even I'm not cynical enough to believe such a silly and childish character could rise to the rank of major in the army, especially in that era. Or that a woman like Margaret Houlihan would have anything but contempt for him.
M*A*S*H had incredibly good luck with its cast replacements over the years. Bad replacements for Frank Burns, Henry Blake, Trapper or B.J. could have really hurt the show, but they stuck the landing every time.