My last long trip with the bumper mounted bike rack was with four bikes, the bumper mount spare tire AND a bumper mounted grill bracket from Atlanta to Savanah down I-16, which is about the bounciest road I know of.
Nothing broke and there were no cracks in any welds, etc.
I think the bumpers and welds vary from one unit to the next, but if you know welds and can rig something to limit the torsional twisting of the bumper, you can safely put a fair amount back there.
I would have no issues using this setup locally again, but for long trips, I put the stuff in the bed of the truck.
2013 Winnebago 2301BH-Red
2012 Ram 2500 Megacab HO CTD