I went to a UU church at times, as an elementary-school age kid. Not regularly, only on occasion. UU seems to be a popular “compromise” religion for mixed families with 1 Jewish parent and 1 Catholic or Protestant parent, who can’t decide how to raise the kids.
Can’t add much about their beliefs(or lack of) beyond whats already been said here. They have symbols from a lot of religions: Western (Christianity, Judaism, Islam) and Eastern (Yin Yang symbols, etc) in the santuary part of the church. The one I went to did have some kind of Christmas choir program (probably secular, can’t remember). The Sunday school is secular humanist stuff, and yes, UU is associated with liberal political causes. That’s about all I can remember, haven’t been in or near a UU place in 25 years now.