With all due respect, your question is inherently laden with false assumptions, which make its answering impossible (without granting false assumptions). As such this won't be an answer proper.
For instance, Unitarianism (the beleif that God is one person) is not synonymous with Monotheism (the belief in one God), because Unitarianism refers to the doctrine that God is one person, not that there is one God only, which both Unitarianism and Trinitarianism - both monotheistic doctrines of God - posit. Additionally, the doctrine of the trinity is not synonymous with tri-theism (if for no other reason than that the trinity doctrine is false if there are more than the one true God - similar to how the incarnate is a false doctrine if God is in fact a man). One cannot deny this without strawmanning the doctrine of the trinity or the incarnation.
Second, Unitarianism goes farther back than the Reformation to the times of Marcion ('Old Testament God is evil - Jesus is the one true God') and the Sabellians ('the Father, Son and Holy Ghost are only three manifestations of the one person of God').
Third, Arianism did not view Jesus as 'just a cool bro' (a good teacher/Rabbi/Saviour) but merely denied that His generation from God the Father was from all eternity, but still placing Him before all creatures, and yet somehow after eternity. They definitely viewed Him as God, or at the very, very least, god beyond all other creatures (including angels). What orthodox Christians were holding a Council over was the fact that they said the Saviour and subject of the entire New Covenant was a creature, although ascribing worship in practice to Him, via the liturgy (especially the Eucharist) etc. They accepted Nicene formulations, with Arian understandings, for example. Arians had no predecessors in the Christian faith - no one ever denied what they novelly - 200 years after Christ - denied.
All forms of Christians until Calvin also denied the presdestination we are familiar with in Calvinism (post 1600s). Predesintination until this point referred to the absolute surety with which God knew humans would behave, and thus end up, and to the surety with which this cannot (therefore) be any other way than it factually was, is and will be. It's just the doctrine that God knows the end already from the beginning, not that God predetermines the end by the beginning point (Isa. 46:10). Foreknowledge does not require God 'rigging' history, in other words.
Denying original sin is as simple as follows:
John 3:5 Jesus answered: Amen, amen I say to thee, unless a man be born again of water and the Holy Ghost, he cannot enter into the kingdom of God.
If you do not have need of rebirth spiritually, then original sin is false. If that's not true, original sin is true - for that's all that has been meant by original sin.
Thirdly, Unitarianism is too wide a denominational swathe to determine that they all put "[low] emphasis on belief." And even if they did, what integrity has the group which claims that the New Testament is false in saying belief is at the heart of the gospel of Jesus? None at all - even if Christianity is false or reprehensible.