Unknown unknowns
GMO, with its large increase in potential interventions, is quite revolutionary.
Cross breeding occurs slowly and is a long-used process. The long-used means there is ample evidence suggesting cross-breeding results in safe food.
The fact that cross breeding happens slowly gives any safety issues a long time to reveal themselves before it becomes wide-spread. Thus it makes sense that cross-breeding yields fewer unkown issues by the time its widespread. Essentially, early warning are built in, even for latent issues.
As an aside, I admire the HN crowd reasoning capacity, but it always amazes me that almost no one is upfront and say we as a society love the taste of beef and are willing to risk environmental and possibly health issues for it. It would be a much more productive discussion if we were honest about it instead of discussing whether we need bigger cows.
These principles then provide the wise sounding argument that our Motivated Reasoning brain center needed.
Personally I'm reasonably confident that any significant adverse health effect would have been found by experiments by now, so Bayesian reasoning tells me any unknowns that are still unknown are probably negligible.