I'm not making any judgments about a particular hitch design or fabrication job, but I do have a suggestion to address this issue: if in doubt, build one and test it according to the published standards, and if it passes, build another one to use (since the test will likely be destructive).
The applicable standard for ball couplers is SAE J684. I don't know which standard would be applied to other types (such as the U-joints and pintle-and-ring), but the same strength requirement as J684 seems like a good start. This standard calls for the hitch to withstand...
a force equal to 1.5 times the trailer weight (the whole trailer, not just the tongue weight) in the longitudinal direction (front-to-back, as in pulling or braking the trailer)
a force equal to 0.5 times the trailer weight in the lateral direction (side-to-side, as in steering the trailer)
a force equal to 0.5 times the trailer weight in the vertical direction (as in holding up the tongue, but it applies in the opposite direction as well)
For details, including test methods, I suggest doing what I did and reading the standard in the reference section of the local university or college engineering library.
I noticed in an earlier topic that these same forces were specified for tongue strength in an Australian standard, which seems nicely coordinated.