I bought something similar (same product, but different brand) a few years back. I never installed it because - as you mentioned- the bumper is not very rigid and my license plate was just screwed on through the plastic. To make the mount more secure, it seemed better to bolt it on but the access behind the bumper is limited unless you remove it. The mount seemed like a solid product other than that.
My only other small criticism of the mount is that it's made for Aussie license plate. The screw holes are father apart than US plates, so the holes in the mount didn't line up to my LC. I would've needed to drill other holes, which was harder than it sounds because of the mount design. Perhaps yours doesn't have this spacing problem.
