While anything can fail I have yet had a CF rod with JB weld fail no matter the length.
The shortest ones I have are basicly two titanium rod ends JB welded together with a CF rod over them.
On my Cap Maniac I used very small ones on the ailerons and flew this airplane for a long time and the rods
performed flawlessly. I ended up cutting down one of the small peices off of the end of the rod ends and then applied
JB weld to both of the rod ends and inserted them into the small peice of CF tube. So basicly the rod ends are JB welded
together along with the CF rod to both of them. This actually could be stronger than a large piece of CF rod that the
rod end is only JB welded to the end of the CF rod and all the parts hooked to each other.
Here is a picture of the wing servo on the Cap Maniac.
John