Poly urethane is intended to bend like that... so when you are in a parking lot and drive into a pole, the bumper bends back into shape. Urethane body parts are more durable than fiberglass parts. Because they are made of a rubbery material they can flex and bend without cracking or breaking.
Fiberglass body kit parts, regardless of the quality, can crack or break with a strong enough impact. Please note that this same impact would likely damage a factory bumper or metal body part as well. Fiberglass will crack, metal will dent, and urethane will rip - all of which will need to be repaired. (if you plan on crashing your car alot, you shouldn't modify your car).
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