Strictly speaking, Unitarianism simply refers the idea that God is one Person, and rejects the Trinity. In general, it rejects the idea of any kind of diversity in God.
But ideas in theology quickly extend to metaphysics in general. If God has no parts, then the creation God made has no parts; there is no fundamental principle to account for the existence of diversity. Hence Unitarianism leads to (and may logically imply) metaphysical monism. It has no explanation for the diversity we see in the world; no way to define categories or to distinguish good from evil.