I wouldn't say that cf is typically either stiff or compliant. Like you say, it can be a lot of things. But there's more to ride quality than that stiffness vs compliance if you ask me. To me the most consistent difference in materials is a sort of harmonic quality. The stiffness or compliance of the whole frame isn't the only thing one senses. I'm not saying it's necessarily important, and I have to admit it may make no difference in mechanical performance, but it's something that makes a difference in my enjoyment. Steel does compliance well and naturally, and aluminum does stiffness better, I think, (but not more durably) setting aside what glue might do to help. CF might do both equally well, but I haven't found it capable of singing.