I had someone recently ask me my thoughts on the Oneness Pentecostal movement, and whether or not I thought them to be Christians. Coincidentally, I was asked to speak at the Prophecy Watcher's conference in Colorado Springs in May, and my topic is on the book of Jude. Thus, in my research of Jude, I'm making mention of the heresies and apostasies that have arisen during the age of the Church, and this idea (anti-Trinitarian) is a major part of it. However, the short answer is I don't believe them to be an orthodox Christian movement. There might be well-meaning but misguided Christians who are trapped inside this movement (similar to converts to Roman Catholicism or Mormonism), but I don't believe the movement itself to be orthodox Christian. On the grade of heresy, I consider it on par with Full Preterists simply due to the amount of Scripture that has to be denied or corrupted in order to be able to hold that view.
So what is heresy? First, heresy (in and of itself) is not unique to Christianity. Most belief systems and philosophies have schisms and in those schisms, the opposing sides view the other as heretical. For example, the Sunni Muslims consider the Shia Muslims to be heretics (and vice versa). However, speaking specifically to Christianity, heresy comes about either by an internal or external influence to explain away some (or a lot) of Scripture through either human or unbiblical reasoning. This change (or corruption) of the Biblical position, then moves from heresy to heterodoxy, and then to heretical dogma.
The early church (pre-70AD) contended mostly with the Judaizers for the reason that Christianity was still largely centered around Israel, and/or Jewish communities outside of Israel. Where the Gospel spread into Gentile communities, the obvious contention then began with pagan and pagan philosophies. After the Romans destroyed Jerusalem, the Temple, and began the Diaspora in AD70, Christianity was thrust entirely into the Gentile Domain so we see the transition from contentions with Judaism, to contentions with Paganism.
The early "Christian" heresies largely fell into three camps: works-based salvation (Judaism, asceticism, antinomianism, etc.), Christological/Trinitarian issues, and Gnosticism. Oneness Pentecostals can sort of trace their roots back to an ancient heresy called Sabellianism (est. 190AD) which was "A unitarian heresy which claimed that Father, Son, and Holy Spirit are merely three distinct characterizatons of a unitarian God as opposed to the orthodox trinitarian view of God being three distinct personages."
The major trinitarian heresies were: Modalism (one God who goes through different modes or roles), Arianism which was "The heresy that rejected the divinity of Jesus and taught that Jesus was created by the Father and that the title "Son of God" was merely given to Jesus as a courtesy. It was condemned at the First Council of Nicaea in A.D. 325." Others were Apollinarianism (350AD) and Nestorianism (428AD) both of which denied some or part of the Trinitarian view. There are many, many more, but you get the point. Since the Church began, many groups have tried to explain, change, or alter the biblical view of God to fit something they promoted. This site has it all laid out quite succinctly (see https://www.godscholar.com/christian-heresies/)
So the issue with anti-Trinitarian positions is not new. I've read through some websites from the United Church of God and other anti-trinitarian groups who posit some supposed biblical "issues" with why the Trinity isn't biblical. Their biggest issue is that they believe that the Bible has been altered much later to insert "trinitarian" verses by the Roman Catholic Church and that Trinitarianism, didn't even exist until Constantine in the 300s. These views are laughable because they are a) historically inaccurate, and b) give a low view of God's ability to preserve His word. Here is an excellent video walking you through the history of Christian heresies that I found both compelling to watch, and easy to learn from.
Hope this helps, and as always, my job is to help you grow and learn in the knowledge of our Lord and Savior!