The bracing is only part of the overall picture. There are many today using carbon/balsa wood combinations and the honeycomb composites in there bracing systems, mostly in the classical guitar world. Greg Smallman being the most successful builder of classical guitars, who start doing this stuff in the late 80's. There are however some compromises along the way, the biggest being the kind of sound some of these instruments deliver. To my ear, some of these guitars have a metallic edge to the sound which I do not like much. With the added strength of the carbon fiber and thinner tops, the sound changes. Some like it, some don't. After years of listening to classical guitar, I prefer a woody, clear strong Spanish sound.
The ukulele has had a few builders working with some of these same ideas, but with a small body instrument, I personally don't hear any advantage for all the extra work involved. This could be a lengthy discussion because of all the different ideas used. Excellent quality Sitka spruce bracing is very hard to beat when it comes to stiffness, weight, longevity, and availability. And remember this, the braces do not make the sound, they only help to regulate and support the top wood which does make the sound. All things being equal in weight and size when carbon fiber or spruce bracing is used, for me spruce wins.