what i'm more worried about is if someone was trying to be an asshole and stomped down on one of them. would they break off pretty easy?
just as if someone had stomped down on the lower front lip of your car, or your exhaust tail pipe or had tossed a tire onto your Hood
or sat on your side view Mirrors
or drive into the side of your car or opened their door onto your paneling or dropped eggs onto your paint or filled your key holes with super glue or put spray paint all over your glass and rubber molding or any other malicious thing... NOTHING is permanent when people want to vandalize another persons property
To answer your question... it all depends on how strong your bumper is - these use metal pieces to attach, to the likelyness of the actual canard breaking, or the metal breaking, is HIGHLY unlikely... but if you kick your fiberglass bumper, the fiberglass will break - and if you attach something to your fiberglass bumper such as a front grill and then kick that front grill, it may also tear your fiberglass...
I'm no rocket scientist, but if I wanted to destroy something on another persons car, it wouldn't be hard for me to do it... I just make sure to avoid putting my personal property in situations that are just asking for others to come and play