Assuming good management in all cases, I think it depends not only on the breed but on the particular hatch as well. I've heard of folks getting a bum batch of chicks from many different breeds. It happened to me with Delawares; I doubt I'll ever get Delawares again. The New Hampshire Reds in the same order of chicks did great, so I'm convinced it was that particular batch of Delawares. I'm not a huge fan of sex-links either. I think they are designed for commercial production and burn out after 2 years or so, and can also develop health problems as they get older, more so than heritage breeds. But again I think that depends a lot on the particular batch, hatchery, breeding lines, etc. I'm sure some are more "engineered" than others, if that's the right word. I am intrigued by the Sapphire Gems and might try some next time I get new chicks. Maybe next spring and hopefully this coronavirus crap will also be over before then.