I disagree, respectfully so. The general anatomy and physiology of chicken is not dependent upon breed of chicken. Please share where I can read where a chicken is not a chicken- G. gallus is G. gallus despite genetic variations that determine traits. Man cannot create a species but man can manipulate genetics to alter traits in future generations; so can environment for that matter.Unrelated, but we literally can't take those studies on broilers as advice for non broilers. Their bodies are not like other chicken's bodies.
Studies on broiler health and nutrition are logically more prevalent. Broilers are highly consumable (literally and expendably) with far more stakeholders gaining benefit from that research. Backyard flock studies lack the population numbers, rapid turn over in lifespan and cost effectiveness for a comparable study to broilers. Moreover, there are simply too many variables in environment to study breed by breed in the quantity required to suit the average BY chicken hobbyist. In other words, it is not feasible so one is left with no generalizable comparison.
I think many will agree that a well designed study following principles of the scientific process (on broilers in this case) trumps anecdotal evidence left to chance- literally.