UU is small, eccentric, and shrinking. Its pluralism, including the inclusion of atheists and agnostics, lack of central theology, and increasingly left politics make it the perfect church for a small minority of people, but too fringe and politically narrow for most Americans.
Cycleback, David. Against Illiberalism: A critique of illiberal trends in liberal institutions, with a focus on Unitarian Universalism (p. 14). Center for Artifact Studies. Kindle Edition.
Unitarian Universalism is a religion without a mission. It has turned to social justice as its major preoccupation without grounding in any coherent theology.
The function of the church in our contemporary society is to enhance the level of spiritual intelligence in its members and the community in which they live.
Unitarian Universalism which could be a conduit for the perennial philosophy has been hijacked by social justice issues of special interest groups and thereby failed in its mission. As these special interest groups have taken over the leadership of the Unitarian Universalist Association it has engaged in policies that are even more detrimental to its function and therefore the denomination has continued to lose members and become even more marginalized in Western Civilization.
Continuing on its downward spiral Unitarian Universalism will eventually disappear. It remains to be seen what will take its place.