The concern, I think, that it's obviously unethical to grow a 100% human child and then harvest its heart, right? You're talking about a living, feeling, thinking person! Causing them to suffer or to die is morally wrong, conversely, their life and their right to life have precious, incalculable, sacred value. The thought of harvesting, murdering a human being for one of their organs is repulsive, harvesting them for food is cannibalism and a taboo of the highest order.

And yet many humans eat bacon for breakfast without a second thought. On the operating table, the human heart and pig heart would probably be indistinguishable to a layperson, so why is the one sacred and the other free for the taking?