I've used mine plenty of times, and haven't had a problem with "flimsy" bumpers at all. Last winter we got hung up in my cherokee when we slid off a trail into deep snow. We just jacked it up with the hi lift and pushed it sideways off of the jack to move it back onto the trail. It took several repetitions , but the bumper sustained zero damage. The bumper on my YJ seems a heck of a lot stouter than the one on my cherokee.

I'm leaning towards mounting the 4xrac on top of my bumper, up front. I just need to make sure I can get the mounting plates to fit o.k. on top of the bumper. Hopefully I won't have to remove the front bumper to mount the rack.
I've also considered mounting it on top of my hard top, but I'm reluctant to drill holes in my top to mount the 4xrac.