EDIT: for some context on the problems this creates
The science is clear that fast-growing chickens like the Ross 308 are doomed by their genetics. These have been engineered to grow so incredibly fast, and their bodies just cannot handle it.”
Jackson said secret filming at broiler farms supplying big supermarkets has shown birds struggling to walk or collapsing under their own weight, or dying from heart failure, and dead birds were filmed lying among the flocks.
[...]
Andrew Knight, a professor of animal welfare and ethics at the University of Winchester, said: “With these really rapid growth rates, it can be difficult for the heart and circulatory system to keep up with the expanding body mass. A proportion of these animals suffer from heart failure. It’s also difficult for the bones, ligaments and tendons to keep up with the rapidly increasing body mass, meaning that a proportion of these birds become severely lame [inability to walk properly].”
And that quote only lists just some of the health problems they face. There's a ton of other problems too
I've been listening to a really good show on this recently. It's not as clear cut as I once thought. Yes, welfare wise these chickens are worse, but environmentally, they are much better. They eat less, poo less and the poo is contained (living indoors) so it can be burnt to create heat for the sheds in Winter, and then reused as fertiliser. When left to roam free, they poo on the fields, which then runs off and destroys river habitats (see the River Wye).
Worth a listen for those in the UK:
https://www.bbc.co.uk/sounds/play/m001stxt?partner=uk.co.bbc&origin=share-mobile