The above link leads to a paper discussing the fact that UUism tends not to be “generational”. (Translation: the overwhelming majority of those “Raised” within a UU context end p leaving.)
I’ll venture to say that this is because “Unitarian Universalism” is basically a real-life version of the “island of misfit toys”: if you can’t quite bring yourself to be Christian/Wiccan/Jewish/atheist etc. but still feel the need to pretend to “belong” somewhere, then UUism is basically the perfect fit: They don’t really give a shit WHAT you “believe” (if anything) — just so you show up occasionally, and (most importantly) keep the $$$$ coming in.
Don’t get me wrong, here: some of the touchy-feelly truisms/slogans appeal to me (“A free and responsible search for truth”, “the inherent dignity of each individual”, etc.) — but they are not nearly enough to build any kind of “community” around.