The bumper on our 21RBS, like many other 'new and improved' bumpers, are very flimsy and cannot hold much weight.
In my opinion, when transporting bicycles, the problem is not so much a matter of weight, as it is of torsion.
The torsion that a bicycle carrier generates on the bumper causes it to rip away from its welding to the frame. (see welding attachments).
My solution to prevent this ripping was to bind the bumper to the sub-frame with a threaded rod and anchors. (see attachment 0,1,2.)
I have done the mod, and it seems very strong, but I have not been out on the road yet.