If you stuff a Cornish X with as much food as it can hold in an effort to get it bigger faster, and keep it confined to a tiny area so that the meat is less exercised and stays whiter - then yes it will have trouble walking and will have some issues with the heart and liver and maybe lungs.
BUT if you portion the food into reasonable amounts, and encourage the chickens to move around, and give it sunlight and fresh air - then all you have is a huge chicken that matures in 7-9 weeks instead of 6 months.
I bought 25 Cornish X chicks for the first time this past spring. Two died the second day (typical for any breed, also had a Rhode Island Red die the same day). One died about 4 weeks in, when I autopsied it it had tumors, so it was sick from the get-go. And one disappeared, I assume a predator got it. The remaining 21 lived just fine through to butchering which happened in small amounts between 7-10 weeks of age. Only one had any difficulties, and she just struggled with standing up after sitting down for a while, so I made sure to include her in the next processing batch, so she only struggled for a day.
It's all in how you raise them. The quality of the food, the amount of food, the quality of the environment. I restricted feed after they were 4 weeks of age or so, kept the water and feed far apart to encourage walking back and forth, and raised them with layer chicks so they'd see their peers running around and imitate them. I used some fermented feed, in the future I'll use more of that. They have a nice life as long as you work to make it so.