I had one welded to the rear frame and had zero issues with the welds or the bumper.
However I did have a rack that the bikes hung from the bike's frame cross member and the bike wheels dangled. Even with all the anti sway items that came with the rack and strapping to keep the bikes apart, they really got beat up from all the bouncing.
My advice is if you go this route, get a rack that the bike wheels rest on a rail so the can be tied down firmly to eliminate the bikes from hitting each other. When I traded tarilers I bought an Arvika rack that mounts over the propane tanks and I'll never have another rear mount no matter how strong they might be.