Heterosis comes with some negative effects, particularly for chickens. Excessive egg laying "wears out" chickens to the point they do not produce enough eggs to justify feed costs. Even if the parent lines do not show early failure, the hybrid can and often does. This generally does not apply to heritage breeds, but it very much does apply to commercial egg production hybrids.
Regarding feeding corn, it is deficient in Methionine and Lysine. I grow a high methionine corn specifically to feed to my chickens. I have fed pure corn to laying hens for several weeks (until I ran out of corn) with no negative effects on the chickens. The point is that virtually all commercial corn is deficient in necessary proteins, but you can grow your own corn that has what chickens need. Sandhill Preservation carries it listed as "chicken feed" corn.