The few I’ve met aren’t Christian and don’t especially like Christianity.
You are correct, to a point. Unitarians may or may not be Christian.
There are entire Unitarian congregations which are indeed Christian and, in fact, some even offer baptisms.
But there are some fundamental differences between UUs and Christians as well.
Unitarian Universalism is an outgrowth of the original Protestant enclave in the U.S., the Puritans. The congregationalist beliefs of the Puritans resulted in the spread of congregationalists churches throughout New England. Then, a schism led to two distinct belief systems evolving - the Unitarians and the United Church of Christ.
Unitarians are not trinitarians. In other words, Unitarians traditionally view God as a single being, not a part of a trinity, which is where the name came from.
The second part of the name, Universalist, derives from the belief that God loves everyone equally, that his love is universal. Therefore, nobody will be condemned to hell. In fact, it would be fair to say many UUs don’t believe in hell.
Because Unitarian Universalist Churches are non-creedal, they are indeed open to people of all faiths, including Sikhs, Hindus, Jews, Muslims, Buddhists, Wiccans, etc.
As I said, some are even Christian, although they would probably be labeled liberal Christians. The same would probably be true of the other mainstream monotheistic religions, Jews and Muslims.
The reason for this is the same reason for Soldier X’s comment regarding UUs not liking Christians.
Conservative Christians, Muslims or Jews would probably condemn Unitarians for their liberal outlook, their use of humanistic teachings and their belief in science and learning, rather than deriving all beliefs from a single religious source such as the Bible or Quoran.
Also, many UUs were raised as Christians, then converted sometime later in life. Often these are people who felt ostracized, excluded or disenfranchised in their “cradle” church.
Whatever the reason, these people have no wish for the Christian majority to try to convert them to their beliefs, which Christians and Muslims seem want to do.
The combination of the above sometimes leads to less than amiable feelings from Unitarians toward Christians.
I don’t personally condone it, because it contributes nothing to our society or out discussion. If I felt as they do toward Christians, I wouldn’t be here.
Anyway, that, in a long-winded way

, kind of gives an overview of how we came to be where we are today.
Cheers