PU (Polyurethane) material, is a flexible plastic. therefore it's very hard for it to actually crack when you scrape it or get hit.

Fibreglass. the easiest to manufacture. most bodykits use this. its cheaper to buy compared to PU type of material, or ABS plastic (most cases).
but the downside is a little heavier and easily cracked when scraped or got hit.

ABS plastic, most OEM stuffs use this material. its a hard plastic that can withstand quite alot of impact and its pretty light (most mobile phone cases use this material, including calculator) can withstand high heat temperature too without going out of shape.
under some certain impact, it can crack. but not as easily as fibreglass.