Are Unitarians Christians?
Historically, yes. Unitarians were Christians who didn't believe in the trinity. (That's how Isaac Newton gets included.) The Unitarians feared that the trinity was a polytheistic idea that left Christians with multiple gods (I'm inclined to a agree with them on that) and instead emphasized the unity of God so as to preserve God's unity and oneness.
In Newton's time there wasn't any organized Unitarian church to my knowledge, and Newton was always outwardly a member of the Church of England, even though he privately had strongly held views that they would have considered heretical. I don't know when or where an organized Unitarian denomination appeared.
As I understand it, to them, Jesus was a prophet, but no God.
Many probably do think that. Today Unitarians more or less believe anything they like regarding theology. So Unitarians have kind of 'evolved' from being the group of Christians that emphasizes the unity of God, into being the 'anything-goes' denomination. In that sense, they may better be thought of as the descendants of history's 'Deists' (Deism originally meant those who accept natural theology but question revealed theology.)
https://www.uua.org/beliefs/what-we-believe/beliefs/christianity/views-jesus
So today, Unitarians are 'Christians' only by courtesy. I think that most of them do think of themselves as followers of Jesus, but most of them probably do think of Jesus as a somehow uniquely qualified and authoritative human teacher of righteousness.
Many members of the more theologically conservative denominations don't really believe that they are Christian at all, but the Association of Theological Schools still accredits the two Unitarian seminaries. (And the ATS will only recognize Christian or Jewish seminaries.)