Unitarianism and Universalism were originally defined by "heretical" doctrines: anti-trinitarianism and universal salvation. And during the 19th century especially, the disputes between the heretical Unitarians and the heretical Universalists against the orthodox were potent and real issues.
But one of the consequences of modernism is that almost all of Christianity has made the move that Tillich made in that Christian doctrines are now understood to be not as actual descriptions of physical and historical realities, but as a set of metaphors which are interpreted as describing the existential realities of human life. Almost all Christians understand almost all doctrines as subject to existential and psychological reinterpretation.
The liberal and mainline Protestant churches are quite open about what they are doing. And even among the Barthian proclamationists, there lurks the same remove from actual reality. The differences between the liberal Christian and the fundamentalist Christian is that the Liberal says, "Let us live as we though believe that these doctrines are true" and the Fundementalist, "Let us speak, think and act so convincingly that we believe that these doctrines are true, that we ourselves and others forget that they are not."
Back to Unitarian Universalism: if all doctrine is being existentially and pyschologically reinterpreted in a creative, current process, does our supposed differences with Christian orthodoxy matter anymore?
For example, suppose one were to reinterpret the divinity of Jesus as a metaphor for the divinity of all humanity -- it is a quite modern and liberal statement to say that "all are children of God". Does that "all" include Jesus of Nazareth? Is it our position that all men are the sons of God, except Jesus of Nazareth?
Or, to work the other side of the street: If we reinterpret salvation as coming into a healed relationship with God and others and the self, in this world, in this history, does it make any sense to say that "all are saved?" Obviously, there are many people who will not come into that healing? If we make our own heavens and hells, then it is pretty obvious that it is not true that there is "no hell."
What I am saying is that in the current theological climate, any aspect of doctrine, or any Biblical truth, can be and will be creatively reinterpreted to speak to any aspect of our shared existential realities. Almost every sort of Christian preacher does so routinely. UU's, to the extent that they deal with Christian, Jewish or biblical themes, do not do so much differently. We are not, in practice, very heretical, or more precisely, our old heresies matter less than we think they do.
It would be subject for another post, but I think that much of the Christian world also agrees with us that all world religions are cultural productions of particular cultural circumstances, and that no religion is more true than the others. We differ from others in that we are more willing to modify our liturgy and worship practices to reflect this reality.