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?
This is an ethical concern which is so completely removed from reality that it borders on satire. This is why no one takes bioethicists seriously anymore.
If they can grow a pig with a 99% human heart, I don't see why that should be more ethically concerning that harvesting a 100% pig heart.