When my dad would make homemade chicken soup, he would give me the chicken liver and heart to eat as a child. I actually liked the chicken liver which was tender. The heart was a little tough.
My dad loved to cook and was a good cook.
When I was much younger my four sisters and I would fight over who got the feet/legs of the chook, the liver, heart and the gizzard to eat.
The livers I thought were rather powdery and yes, the chicken heart and gizzard were "tough" to chew on, but that was dependant on how long they were pressure cooked. There was also a number of handfuls of barley in the pot as well to chew on.
I have observed that reheating the chicken soup/stew in the microwave also helps to soften the hearts and gizzards. We must remember that the chicken hearts and gizzards were the hardest working muscles in the chook and as such they were not as tender as the rest of the meet on the cook carcase.
Another delicacy I enjoyed was calf liver, but it could not be overcooked, otherwise it too would become powdery to eat. I still cook liver, bacon and onion in a large frying pan and after deglazing the pan, make finger licking good gravy from what comes off of the pan. But these days even calf livers are difficult to come by. Sadly, these days to many cooks turn the bacon into a carboard like treat to eat. I like my bacon to be soft and tender to eat.
I also like kangaroo meat but if that is overcooked, it too becomes tough to chew upon. I cook the kangaroo meat by searing it on both sides to give it colour and then finishing the cooking off in a microwave so that it retains it moisture and tenderness.
Oh well to many secrets that i should not spill.