I have the plastic version, note that it isn't fibreglass so it has a degree of elasticity which helps.
It does sometimes scrape on the odd speed hump (not all) and steep car park ramps can be "interesting"! I can feel a few minors scrapes but are all underneath (obviously!) and not visible.
As for parking facing a kerb, yes you could damage it but you get used to keeping a bit of distance.
Get one!
David