I don’t consider Unitarians as Christian or Protestant.
Historically, Unitarianism and the Unitarian Church grew out of the Protestant Reformation of the 16th century.
Unitarian churches were formally established in Transylvania and Poland (by the Socinians) in the second half of the 16th Century.
In England, the unitarian movement gained ground in the 18th Century when Samuel Clarke revised the Book of Common Prayer, removing the Trinitarian Nicene Crede and references to Jesus as God. Theophilus Lindsey also revived the Book of Common Prayer to allow a more Unitarian interpretation. His efforts met with substantial criticism. In response, in 1774, Lindsey founded the Essex Street Chapel, the first true Unitarian congregation in England. A third Anglican, Joseph Priestly founded a reform congregation. Priestly fled to America after his home was torched, and became a leading figure in the founding of the church on American soil.
In the United States, the Unitarian movement began primarily in the Congregational parish churches of New England. These churches, which may still be seen today in nearly every New England town square, trace their roots to the division of the Puritan colonies into parishes for the administration of their religious needs.
So I would consider Unitarian to basically fit within the Protestant wing of Christianity, given that Unitarian churches were created as offshoots of more mainstream Protestant movements.
That being said, I have no doubt that most Protestants overwhemingly reject Unitarian theology and were swift to denounce the rise of Unitarian churches as not following true Christian doctrine. So while most Catholics and Protestants today would reject unitarian teachings as non-Christian, the entire Universal-Unitarian movement traces its roots to Protestant Christian churches.