A given structure made from advanced composite materials is significantly lighter but more solid and rigid at the same time than a comparable aluminium structure because of its higher tensile strength.
For Hovercraft this translates into a higher power-to-weight ratio (better payload capacity, performance and higher seaworthyness).
Aluminium is only available in flat sheets of standard sizes and thickness. It needs to be welded or rivited which results in areas of high stress and structural weaknesses. In addition, aluminium has a low tensile strength compared with advanced composites. These factors result in aluminium structures being more than 20% heavier than a comparable structure made from advanced composite material.