Carbon fiber is an interesting material. It's extremely rigid, so while it may be strong in some ways (like warding off direct impacts), there's not much tolerance for bending, so it can break under the right stress pretty easily. Take the Sony Qualia 010 which I mentioned above. It was the first headphone to have a carbon fiber frame. This made it lightweight---disturbingly so---and extremely comfortable, but the headband especially was prone to snapping in two. A carbon fiber shell on a CIEM however would probably be a much more durable proposition given its shape compared to a headband which, by its very nature, needs to flex quite a bit. However I'm not sure the benefits in this particular application would be that much better than acrylic, which is the material from which most shells are made. When you describe the failure of your JH13Pro, do you mean to say it was the cable that failed? I'm not seeing how a carbon fiber shell would prevent a cable failure like that, though the new locking mechanism JHA is implementing looks more sturdy and might do just that. Either way, regarding shell material, I'm not so sure a more rigid material is better when it comes to something I'm sticking in my ears. Silicon shells for instance tend to be regarded as more comfortable.
Of course, this is all assuming these shells are even made purely from carbon fiber. I haven't seen anything so far to indicate the carbon fiber shells aren't just decorative. That is to say, that it's not just a layer of carbon fiber covered by acrylic or resin and instead 100% carbon fiber. If it's just decorative, then there would really be no genuine benefit I can fathom. To that end, my main point with the whole carbon fiber thing above was purely aesthetic. To me it just gives off this slightly cheesy pseudo-performance vibe, especially when applied to stuff like shifter knobs, fountain pens, and ear pieces. I just want to go "vrrrroooom! vrrrrroooom!" when I see it. Then again, I'm the sort of person who finds slightly cheesy things endearing. So it's actually not a strike against them in my book LOL.
Another point I sort of implied above---one that actually IS a strike against them in my book---is that if JHA gives people shells with jagged acrylic, I hope they do a better job with the carbon fiber which is a very tricky material to work with, far more so than acrylic AFAIK.