Lower stiffness of the Blades, higher tendency to twist.
I think all helicopter pilots of my background are aware of that - we all got to see the film clip of the Whirlwind blade taken from a camera on the rotor head many years ago.
But what point are you trying to make by the above statement? You said that older designs of MR blades changed in profile along their length. I pointed out that that wasn't always the case.