The thermal properties of fiberglass are not the same as steel. It heats up and cools much faster thus, it expands and contracts at a different rate. This affects the bond when exposed to rapid heat, cooling and vibration, and there’s the key: Bond. Fiberglass repairs are only as good as the bond. Welding takes two pieces of steel and effectively makes them one. For what it’s worth, if I were replacing rusted metal and couldn’t weld, I would buy a steel repair panel, flange the edge of the area needing repair, and bond it using panel bonding adhesive. If you live in the north, don’t use glass.