I noticed with my dog that if she has not eaten liver for a while she will eat loads of it, but once she's done pigging out, then she will reject it for a while, or only accept a bite or two. There is such thing as too much of even a true superfood like this, and when you consider that too much of a polar bear's liver can supposedly kill you it's not surprising.
I've never really eaten it in large enough quantities to feel at the upper limits, but I see a definite improvement in the way I feel when consuming even supplementary amounts of it. Even cooked it's got plenty of energy to give. Everyone should eat liver, that is my opinion for sure.
I have wondered though too if maybe kidney wasn't just as special as liver, or pancreas. I don't have as ready of access to these as I do liver so more experimentation necessary! yum yum!