My experience with aftermarket bumpers has always been this;
Aftermarket aluminum bumpers are made much stronger than the factory steel bumpers.
Hit something hard with the factory bumper, and the bumper will bend, usually saving the frame.
Hit something hard with a quality aftermarket bumper and the bumper will be fine...and transfer most of the impact to the frame, usually bending or damaging the frame.
Not always, but I have seen it happen and done it on several occasions.
I like a bumper for it's looks only for that reason.
Just my two cents, for whatever that's worth.