The underlying problem with the UUA was probably found in its cradle. It is certainly present in its mature middle-age. Who are we, at last? What makes us distinct, so as to identify boundaries, however generous? That's never been clear. While that should be a hallmark of tolerance and acceptance, it's been my experience that it breeds suspicion and resentment. Why are those Christians still here? It also creates confusion as to our mission. That's a description, not a judgment.
It doesn't matter if the Unitarian Univeralism is (or tried to be) "America's third religion" or "America's real religion" or "the religion of the future" (early slogans) or "the religion that puts it's faith in you" (more recent) or any number of identifiers used by leading preachers or a Boston publicity office in the last six decades. Each of these ways of identifying our religious fellowship have been majority identifiers, without mandate or power to rule out those who happen to be in the minority at the moment. Which has helped me; I've never been in the Unitarian Universalist theological majority. Even now, Ken Patton's multireligious experiments just make me roll my eyes. But that doesn't mean I'm less a part of the community, at least not formally.
By definition, most of my readers will be satisfied with how Unitarian Universalism is today. Or if you don't like who's ascendant, just wait a while: you may live long enough to see the majority opinion wither away. The Christians gave up hope for a majority decades, and the once-jubilant Humanists are feeling the pinch now.
So in practice, it means that we have maintained a breadth of expression that is hard to distinguish from society at large, or in recent years the Left side of it? And that means being doomed to follow culture rather than leading it. Perversely, were there two, three or four smaller related liberal denominations, each could be a bit more distinct and have its own genius or charism. At least it could have its own voice.
But that's not what we have, and that's the problem we have to manage.