Camping buddy carries bikes the way you mention. Has done it for 2 years, probably 6 trips (2000 miles) with no issues. On our last trip together, last week, you could start to see a little twist in the bumper developing. He is now concerned and considering welding a proper receiver to the frame, but the bumper has not failed.
With bikes the issue is dynamic force from the natural trailer bounce when towing. A static 100lbs is totally different than the dynamic force from (2) 25 lb bikes plus (1) 20 lb bike rack hanging from the bumper.
I opted for the frame to frame welded receiver. It cost ~$300 if I recall correctly. I have done it to both my current TT and previous. Neither have had any issues hauling 4 bikes.
I think it's just a gamble. If you are willing to take it with the bumper mounted receiver just keep a close eye on it. I would use straps to try to minimize as much bouncing as possible.
The roads you travel will have a lot of affect too. If you typically travel smooth well surfaced lanes you will be much better off than rougher roads.
As I previously said, I didn't want to take the gamble but many do. Some will have issues, others don't. I hope you are successful, but at the same time I would research what it would take to have a receiver fabricated and welded too. It might be affordable enough for you to not take the gamble, that is what I found. Ours was done at a local hitch and muffler shop.
2007 Chevy Duramax LMM/Allison
2012 Jay Flight 29QBH
2012 Jay Flight 26BH (Sold)