Composite or fiberglass shells sit between polycarbonate and carbon fiber when it comes to weight and durability. They offer a solid balance — tougher on the surface than polycarbonate but lighter than ABS plastic. However, you won't find many recreational ski helmets using this material.
Where fiberglass shells shine is in racing and mountaineering applications. Racers appreciate that the material handles repeated high-speed impacts while keeping weight reasonable. Mountaineers sometimes prefer composite shells for durability in rough, rocky terrain where a helmet takes more abuse than typical resort skiing.
For most resort skiers and snowboarders, composite shells don't offer a compelling advantage over polycarbonate, which is lighter and more widely available at similar price points. Unless you're racing competitively or doing serious alpine mountaineering, you're better off sticking with the more common shell materials.