When using a bike rack on the rear of a trailer, there are at least two things that can go wrong: the attachment of the rack to the trailer, and failure of the rack itself.
My first fifth wheel was a Sunnybrook. I bought an receiver attachment that just clamped to the bumper. The rack was built for two bikes, and was a brand name (sorry, don't remember which). I put two bikes on the rack, and went on a trip. After several hundred miles, I was alerted by a motorist to a problem.
What I found was that the rack had broken and was dragging both bikes on the ground. The receiver attachment to the bumper and the bumper were still solid - just the rack failed. One of the bikes was severely damaged. The manufacturer of the rack said that standard bike racks are not recommended for use on the rear of a trailer, due to oscillations of the trailer when traveling.
This was all bad enough, but we were really lucky that the rack and bikes remained attached by cables I used to lock the bikes to the rack. If two bikes and the rack had fallen loose from the trailer and ended up on the roadway in front of highway traffic, well... it could have been bad.
As they say, YMMV.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch