It sounds like they ate them right away. This makes them tough. You need to let them "age" in the refridgerator for at least 24 hours, then cook or freeze them.
I'm also wondering if they were meat birds or dual purpose/laying breeds, or if they were older birds. When we have culled from our laying flock, the meat is tougher and darker than the meat breeds. Mostly because the chickens are older. The older the bird the darker and stringier the meat. As for the taste, it could have been the oil gland, or maybe just the difference between storebought and homegrown. They do taste a bit different, but most people prefer the homegrown.
Another thought...if they did not chill the carcass immediately and completely it could have begun to spoil. It really doesn't take long when the meat is warm to begin with.
Anyway, don't let their bad experience put you off raising/eating your homegrown chicken. It is the best you'll ever eat, and gives great satisfaction to know that you are providing the best possible food for your own family.