Carbon fibre composites - and they are safe if applied and produced correctly.
This stuff is used in lightweight constructions because it can take more load per weight then steel.
Other then steel it doesn’t bend or transform. If CFCs are destroyed they crack or even pulverize - this property is used in the manufacturing of racing cars to absorb a lot of energy in case of a crash.
With Airplanes it’s certainly all about the weight and - about the prevention of cracks and the pulverizing part ;-)
The production is quite high techy because the large parts of the fusilage have to be molded with nearly no tolerances - and with no bubbles between the carbon fibre layers. So far there’s no other company capable of doing that. Boeing is believed to have a 5 year head start on airbus with the manufacture of these parts although Boeing didn’t have much experience with composites in jet-liners before this project.
So this is a daring and remarkably global project (2/3 not from the US - wings from japan )
... and the safety regulations for aircraft are quiet strict so certainly it will be safe.
When I worked for Boeing in the late 60s, the wing leading edges on the 747 used composit construction. At the time it was fiberglass and honey comb sandwiches.