I currently have 3 proof barrels in 6.5 SAUM, 7 SAUM, and 300 WSM. I also had one in a 338 Lapua. I haven't had any problems at all with mine. They shoot just as well as a high end steel barrel based on my experience (all easily under 1/2 MOA). Now one thing to keep in mind is that most people that are using Proof Carbon barrels are using them on hunting rifles, which TYPICALLY means lighter rifles, with larger (possibly magnum) cartridges. For example... I personally cannot shoot my 8.5 lb. 300 WSM (with a carbon barrel) as well as I can my 16 lb. 6XC target rifle (with a heavy steel barrel). They both shoot very well, but it's just harder to shoot a lightweight, magnum rifle than a heavy weight target cartridge.
The other thing Carbon barrels excel at is using them with suppressors, especially suppressed hunting rifles. If you are trying to keep the weight down with a steel barrel you have to use a thin barrel, which then flexes more when you add the weight of a suppressor, which can make a significant POI shift between suppressed and unsuppressed shooting. With the stiffer, lighter carbon barrels this POI shift is much less. Also - It can be a challenge to have enough mounting surface on a thin barrel to add a suppressor, so you end up threading the muzzle with non-standard threads like 1/2" or 9/16" diameters. With a carbon wrapped barrel there is always enough steel at the muzzle to support a standard 5/8x24 thread pitch for your suppressor.
