You are spot on there Larry. lengthening a chamber will always produce a thinner area in the barrel wall than what existed previously. As to whether or not that constitutes a hazard depends totally on what was there to begin with. Another factor certainly worthy of consideration is as to whether lengthening a chamber is going to encourage the use of heavier loaded shells.
Lengthening a cone is an altogether different matter. It very seldom produces a thinner area than what is already presently there.
I personally do not recommend either, but both can under some conditions be done with complete safety. A "Competent" gunsmith would evaluate the individual gun & "NOT" make a Blanket statement either For or Agin'it.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra