Since I want to build up a new bike with road or gravel handelbars and aerobars I try to get as much information about it as possible. Now I have just watched a video where the person in it says (at 00:55) that it is not recommended to use aluminium aerobars with carbon handelbars. I am just wondering why that is (maybe because they want to sell more of the cost intensive carbon stuff). Both can make scratches on the handelbars but if you tap it both should not. Any ideas why this should not be recommended?
First, that seems to be generic advice, so there are bound to be exceptions.
But, in general, there's a really good chance carbon-fiber handlebars are not round except for the central clamp area, and in general are not designed to have weight-supporting structures clamped to them.
Aluminum bars tend to be round, and aluminum in general is a lot more forgiving of undesigned loads than carbon fiber might be - any one bit of aluminum will have pretty much the same characteristics (tensile strength, stiffness, etc.) in all directions. Carbon fiber can be extremely anisotropic - different properties when measured in different directions.
Aluminum bars strong enough to handle the shocks of supporting significant weight over bumps pretty much have to be able to handle the pressure of clamp-on load-bearing devices such as aerobars. And aluminum bars tend to be simple tube shapes and not complex forms.
Carbon fiber bars, on the other hand, can easily be complex non-round shapes that aren't readily able to handle non-specific clamp-on accessories. Carbon fiber can also be designed in ways that it can't safely handle significant loads that it wasn't specifically designed for.
So if you get a set of carbon fiber road or gravel bars that are round, and specifically designed to handle clamp-on aerobars, they'll work. Good luck finding such bars, though, as carbon fiber bars tend to be expensive, high-end, dedicated-to-their-specific-discipline accessories.
I suspect that's why that video gave that advice.
Clamping items on Carbon handles have a limited torque amount unless they are designed to take more force. I bought a Mt Zoom MTB handlebar. There was two versions, one for using handlebar ends, and a second version that was lighter but would NOT accept handlebar ends.
I think it more depends on the handlebar, rather than if the material is alloy or carbon. I don't think the material means anything in this context.