>>550226
I don't have experience with carbon fiber handguards, but with compound materials in general.
They are real shit for handguards in theory. You see, compound materials consist of two parts: a maxtrix, which is some plastic or resin, and a fiber, usually carbon, glas or aramid. There are multiple methods to manufacture them, but that's not the point.
The material you get can only really take forces in the direction the fibers are placed, even coming from more than 6° angle will reduce strength by more than 90%. This is why the fibers are woven into a fabric and then placed into the matrix. This way forces can be applied from all directions, but you also increase the thickness (and thus the weight, which is the main advantage of reinforced fibers anyways, so you lose your main advantage).
You CAN save a lot of weight using reinforced fibers, but only if you can predict what forces will be applied from which direction. This is why they are used in cars and planes, but not for the hull, rather for beams on the inside. You can save a lot of weight (and thus fuel, and increase max cargo) that way.
Handguards however must be resilient from forces from all sides and angles. No matter if there is a blow from the right or the rear or from some strange angle inbetween. This means that you must use multiple layers of woven fabric, greatly increasing thickness and weight.
Another thing about reinforced fibers is that it is extremely hard to diagnose them, and they lose strength when they get damaged. So if a blow to the side of a handguard would just put a dent in it if it was made from aluminium, or possibly crack it if it was made from wood, then it would cause tiny fractures and delaminations (two or more layers of fabric breaking apart) in reinforced fibers, which you can't see, and will only notice once the part breaks apart after another couple of blows. Don't get me wrong, I am not saying that reinforced fibers are somehow weaker, but if they get damaged you won't know. With metal you can see the dents with your own eyes, but with fibers you need to employ advanced (and really fucking expensive) ultrasound diagnostic tools.
Overall, they have a reason to be used, but the way we see them used now as a "black metal" doesn't make any sense whatsoever.