Is this really a new topic for you? Arnold's book has been put down on these boards for longer than I've been around. Adino worked with Brother Arnold and assisted his research and has shared many insights.
Just from the top of my head:
He states that "Montanist were catagorized as Sabellians." But in reality Tertullian was a Montanist and wrote against Sabellianism. How could the Montanists be "Sabellians?" I sat in a room with you when a professor at UGST compared the Montanists to AoG. Do you remeber that?
Arnold states "Manichaenism revered Pentecost, Jesus name." He groups them with "other Oneness bodies" including the Bogomils. Manichaenism and Bogomils were both dualists. The didn't even believe that Jesus Christ had a physical body and they believed that the devil was a god on par with Jehovah!
Arnold states that the Bogomils, Albigensians, Cathari and many other medieval groups practiced
Acts 2:38 salvation. But, he tells us, Rome lied about it and burned all of their books. None of that is true. In fact Google will take you to Albigensians texts. Read them for yourself and see if you would have fit in with their beliefs.
He states that "the Cathari - were people of the true Christian Church..." This "true church" forbade its people to marry or to consumate a marriage. They were mostly vegetarians and they believed that Jehovah was the "evil" god who created the material world but that the "son of light" had come to save us all from Jehovah/Yahweh. Is that what you preach?