Fiberglass can be made into many different shapes and sizes fairly easily as compared to metal. Different varieties of resin allow for FRP use in the most drastic environments. For instance, fire-retardant resins can make it so your FRP will not burn.
Expansion and contraction with heat and cold can be extremely important in certain circumstances. When paired with the right resin, FRP can be very durable in a wide range of temperatures, unlike a metal, plastic, or wood.
UV derogation: When paired with the correct resin and gel coat your material won’t crack or disintegrate. Plasti-Fab places UV inhibiters and pigment in their resin which allows the product to keep its structural integrity over long periods of time. One study shows that the tensile strength and tensile modulus loss over 35 years was very minimal.
Acoustical Advantages: When using metal where sound pollution is not acceptable, you’re going to have trouble keeping the sound waves from bouncing off the walls (literally). However, fiberglass tends to absorb sound waves much better, and when paired with additional materials, you can eliminate that pump sound to the outside world completely.