I think the main reason for wanting Cornish genetics in a cross is the breast meat. Other birds my get taller, but not much heavier. A full grown roo can weigh over 10 pounds. They are short, stocky birds, but deceptively heavy. I am currently raising mine with some RIR and BOs that were born at the same time. The latter breeds are taller and look much heavier due to feathering, but they are nowhere near the heft of these things. And the other birds have bony breasts, unlike the Cornish.
I've also kind of decided there is a lot of conflicting information out there regarding them. For instance, the OSU extension office and by extension the American Livestock Breeds Conservancy which uses the OSU information, the Cornish has a difficult time breeding and may need AI. Well, a true standard Cornish has no such problem, as they have been around for well over 100 years. However, I could see how a roo could have difficulty with a much taller hen of another breed.
I understand the Buckeye has some Cornish genetics. I've never seen one, but if those genetics affect the breast meat at all, I would probably want to take a look at them if I wanted a good all-round bird.