“and from bird to bird in a flock” This is the problem with those reviews. Many people know absolutely nothing about basic bio security and you do not know what those birds in the reviews were exposed to outside of the hatchery.
The problem you run into with cancers is there can be genetic predisposition yes, but also environment and husbandry can play a role into things. Not to mention hatcheries sell millions of chickens every year so no matter how great the parent stock is there will always be chickens that end up...