I just read the Humanist Manifesto 1, written in 1933. I found it reasonable and saw nothing in it that should annoy any rational human being. On the other hand the total negation of the god concept might bother a few billion people, but then those people should realize that what they believe lies outside of rationality.
They might also be bothered by the 1st principle in that it claims the universe is self-existing and not created. That is still unknowable, so it is a bit of a quibble if an intelligent willful being set the whole universe going, and a declaration one direction or the other should be accepted by either side as nothing more substantial than a simple assertion. 2nd principle – Humanism believes that man is a part of nature and that he has emerged as a result of a continuous process. That leads directly to the 3rd principle – the traditional dualism of mind and body must be rejected. The humanist implication is that the mind is part of the brain and thus dies when the body dies, and did not exist before the body was formed. That is unacceptable to many because if the mind/spirit vanishes at death then the goal of life and all existence is ultimately meaningless and humans must create their own meaning.
The rest of the 15 principles are derivative, and if there is any problem with accepting the first three there isn’t much reason to pursue the remaining principles. If those first three are seen as reasonable the rest are obvious applications of those ideas, and you should consider reading them for a general clarification of the goals of a humanist life.