Recently, Catholic apologist Gary Michuta interviewed Matthew Swaim on the Catholic devotion to the Sacred Heart of Jesus:
The problem with this devotion is that it is not merely symbolic and is actually Nestorian in theology. For a discussion, Dave "TheRealMedWhite" (an Eastern Orthodox apologist) has a useful video on this:
In this video, we see that "heart" is not being used as a synecdoche for the totality of Jesus or in a symbolic sense--it refers to the actual heart of Jesus.
As an aside, I do find it interesting that Robert Bellarmine (correctly) noted that many of the Protestants in his time were, functionally, Nestorian vis-a-vis the atonement and related issues:
Luther and Calvin favor this heresy not covertly, at least in their way of speaking. For, Luther in his sermon on the Nativity of the Lord said: Some ignorant people make Christ the man omnipotent. But the same Luther often taught the opposite, so that he seems to have been more a Eutychian than a Nestorian. Beza also saw this and made note of it; for, at the end of his book against the thesis of Jacob Andreas, he quotes the sentence of Luther, and then adds that the sins of Germany and the whole world brought it about that Luther did not persevere in that confession.
But Calvin, in book 1 of his Institutes in chapter 13 § 9, when debating about Christ as he is a certain divine person, says this: I have not yet attained the person of the mediator and in § 23 he said: I respond that he is the Son of God, because he is the Word begotten from the Father before the world; for there is not yet a word for us about the person of the mediator. And in § 24: For because of this, Christ was manifested in the flesh. He is called the Son of God, not only inasmuch as the eternal word was begotten from the Father before the world, but because he assumed the person and office of mediator. There Calvin always seems to distinguish in Christ two persons, one of the Son of God, the other of the Mediator. And it seems that he cannot be excused, as if by the name of the person of the mediator he understands not a substance but a quality, in the way in which we are accustomed to say that another person assumes the person of a judge or of a lawyer. For, in the same § 6 he said: I call the person subsistence. Therefore, at least he cannot be excused or a vicious equivocation. But in a very clear way Brentius favors this heresy in his book on the Majesty of the man Christ, where he often repeats that the Son of God is in the Son of Mary; and Smidelinus does the same in his theses. (Controversy 2, book 3, chapter 5 in Robert Bellarmine, Controversies of the Christian Faith [trans. Kenneth Baker; Keep The Faith, 2016]. 472)
