As mentioned fiberglass covers a wide range of materials, from thin roll materials like Filon, to thick solid boat hull like fiberglass. Of course the thicker it is, the heavier it is, and being the roof it is at the top of the coach, potentially making it more top heavy and less stable.
Some fiberglass roofs are basically taped down along the side walls, and this seal must be checked and maintained or else the wind might get under it and peel the entire roof off. So while fiberglass may be more durable than the TPO and EPDM rubber roof material, it is not maintenance free.