Again, some things will be more applicable than others, but in most cases broiler chicken studies will be more useful than cattle or pig studies. Since there are usually no studies on chickens kept in typical backyard flocks, the broiler studies provide more information than the "none" and "a few anecdotes" we would have otherwise.I'm not realy talking about food as much as general health and illness.
As an example, whether fermented feed improves health (usually yes by a small amount) might apply to other breeds as well. But what percent of chickens die from a particular strain of virus may be way off, high or low, as compared with specific other breeds or mixes.
I agree that there are enough differences that the studies should be viewed with caution, not accepted as 100% accurate for all chickens. But I do think they have some use in some cases.