Latter-Day Saints themselves consider the Book of Mormon a New Covenant, as this book notes, that supersedes the covenants of the Hebrew and Christian bibles.
The writings of Brigham Young and other church elders has been quite racist, as is some of the text of the Book of Mormon itself.
he had caused the cursing to come upon them...wherefore, as they were white, and exceedingly fair and delightsome, that they might not be enticing unto my people the Lord God did cause a skin of blackness to come upon them. ~ 2 Nephi 5:21The ban of Blacks from the priesthood because they bore the Mark of Cain was lifted in 1978. See also Deuce of Clubs.
Cain slew his brother....and the Lord put a mark upon him, which is the flat nose and black skin... ~ Brigham Young, Journal of Discourses (third paragraph from the bottom)
Negroes in this life are denied the priesthood; under no circumstances can they hold this delegation of authority from the Almighty....The Negroes are not equal with other races where the receipt of certain spiritual blessings are concerned, particularly the priesthood and the temple blessings that flow there from. ~ Bruce R. McConkie, Mormon Doctrine 1958
When God allows a spirit to take on a Negroid body, do you suppose He is unaware of the fact that he will suffer a social stigma? Therefore, if you say this Church is unjust in not allowing the Negro to bear the Priesthood, you must, to be consistent, likewise say that God is even more unjust in giving him a black skin. ~ John J. Stewart, Mormonism and the Negro, 1960
As for polygamy, the people who currently practice it have been officially excommunicated by the LDS. But, they believe themselves to be Mormon fundamentalists. They believe the ending of polygamy in 1890 was a political act to gain statehood for Utah (it was) and they refuse to follow it.
What is your definition of a cult? (1) Any religion founded by one person who claims unique insight and/or personal contact with God. (2) Any religion that requires its acolytes to cut themselves off from the rest of society like, say, trekking out to the Utah desert in 1847. (3) Any religion founded in antebellum upstate New York or California in the 1960's. Those were weird times and places for religions.