Glass fibers are strong and have a high flexural modulus however glass fibers are brittle  and not fracture-tough. 

Also glass fibers have significant memory and are difficult to adapt to teeth without rebounding back to its natural straight position, accordingly  its efficiency as a splint in the mouth is lost.

UP Fiber Splint made of UHMWP has a fracture toughness,  damage tolerance and is not  stiff and brittle. It also has good manageability, it is possible to be accurate placement  to teeth.