What does that have to do with whether or not they’re heretical?
If you are speaking of Miaphysitism…
St Cyril of Alexandria said that people who “maintained a difference of natures” are “understanding in simple thoughts (one translation says: “simple-minded”)”. He clearly considered “One nature of God the Word Incarnate” to be a more advanced understanding of Christ.
“But the brethren at Antioch, understanding in simple thoughts only those from which Christ is understood to be, have maintained a difference of natures, because, as I said, divinity and humanity are not the same in natural quality, but proclaimed one Son and Christ and Lord as being truly one; they say his person is one, and in no manner do they separate what has been united. Neither do they admit the natural division as the author of the wretched inventions was pleased to think…” - St Cyril of Alexandria, Letter 40:17
I am a Miaphysite. The Catholic Church accepted Miaphysitism as an orthodox explanation of Christ before Vatican II. St Cyril supported it (even after the reunion) above Dyophysitism. And Venerable Pope Pius XII accepted it as an orthodox understanding.
Those who call Miaphysitism heresy need to read the letters of St Cyril of Alexandria.
After the reunion with John of Antioch:
“Therefore, whenever we have these thoughts in no way do we harm the joining into a unity by saying he was of two natures, but after the union, we do not separate the natures from one another, nor do we cut the one and indivisible Son into two sons, but we say that there is one Son, and as the holy Fathers have said, that there is one nature of the Word [of God] made flesh.” - St Cyril of Alexandria, Letter 45:6 (to Succenus, Bishop of Diocaesarea in Isauria)