I used to work in the bodyshop and have seen the cases of what you are talking bout. But this is not on trucks and mostly on SUV/crossovers. Most of the brush guards are just for looks and not really functional. The most noticeable case was on Mercedes ML. It got in to an accident on the passenger side. If he would not had a BG the damage would've been the right side headlight, bumper and a grill. But since he had a bg on it got bended in and got smashed into the hood to the point that it jammed the hood from being open, it got smashed in to the left headlight scratching it and got some dents on the right fender. It definitely multiplied the damage 3 times.
But on that note you have to think about bg as, first it is not designed for preventing car accident but just to prevent from small brushes and some accidental bump-ins(like while you park your car), second the quality makes BIG difference, ones are made for just looks and mostly installed on the cars that are not designed to have it in the first place, and others are very good quality, sturdy and functional that made for the offloading purposes in mind.
No matter how good the brush guard is or even the thor parts bumper will not help you in the case of the high speed collision. Actually having metal bumper is even worst in high speed collision then the conventional plastic one with absorbers and crumble zones.
