you didn't mention how old your relatives birds were when they were processed. an older bird will be tougher, a 3.5-4 pound dressed bird will be tender. that is the reason behind seeing fryers, broilers and roasters at the store (you may already know this, it is not my intent to insult anyone's intelligence).
however, as said, the birds you buy in the store have tenderizers, preservatives and flavor enhancers pumped into them. IMO, the cornish cross won't be as flavorful as a longer-growing bird, anyway, but they still will taste better than that stuff we buy in the store.
if they did not let the bird rest long enough (24-36 hours) it would have been tough, and they would have thought they were being creative and successful in later attempts, when in fact, the bird's muscles just loosened up
we put a 5.5 pound bird on the rotissierie, and it was fine, not tough at all (well, not as mushy as the store bought birds, but it was great)
roast larger birds like a turkey, slow cooker and techniques like that - slow, low and moist heat.
i guess feed would also play a part. we've only ever fed corn, so don't know how it would affect them to feed other stuff
i wouldn't worry about it too much. just process CX's at 8-9 weeks (or, just before they die naturally
- that was a joke), let them rest before freezing or eating, and you'll be fine.