the only thing that worryes me with alu bumpers (at least front) is the crash safety in regular trafic, for racing too especialy rally( trees , lamp posts,...).
the alu bumper would have to be built to the same strength and stiffnes as the steel one.
Letter A
what worries me... the bumper connects the chassis legs so they dont split appart in an even of crash , if you check the original steel bumper you will see that the rear plate(the one with holes for mounting the bumper is thicker and profiled for stiffnes ( like an U section).
if the bumper fails in the crash the next thing stopping the "intruding object that you crashed into" would be subframe , and it is good 500mm behind the bumper. and that is if the object was mounted to the ground, SUV isnt mounted to the ground.
see what i mean,
green chassis legs, blue bumper, red subframe
there is a lot "empty space" behind the front bumper.
letter B
on the other hand on the rear the chassis legs are interconected with the boot floor and spare tire well, and its encased with the sides of the trunk( rear arches ). that part is solid enough by itself.
the properly made alu bumper would have to be designed to the same strength( not stiffnes) as the steel one.