Young's reputation is genuinely disputed, and the disagreement is about substance rather than emphasis. Three matters carry most of it.
The racial teaching is his. Speaking to the Utah territorial legislature in 1852 he stated that men of Black African descent could not hold the priesthood, and the restriction that followed lasted until 1978 and extended to the temple ordinances. The church's 2013 Gospel Topics essay states that it began under Young rather than under Joseph Smith, that no revelation is known to have established it, and that the church disavows the explanations later offered for it.
Plural marriage was announced publicly under him, at Salt Lake City in August 1852, and he practised it on a large scale, marrying some fifty women over his lifetime, with fifty-six children by sixteen of them. The practice defined the church's relationship with the United States for the next four decades and led to the federal legislation and prosecutions that nearly destroyed it.
The Mountain Meadows Massacre of 11 September 1857, in which Latter-day Saint militia with some Paiute participants killed about a hundred and twenty emigrants from Arkansas, occurred during his governorship and in a territory he had put on a war footing against a federal army. The fullest modern account, published by Oxford University Press in 2008 by three historians with unrestricted access to church records, concludes that local leaders planned and carried out the killing and that Young did not order it, while treating the climate he had created as part of the explanation. That conclusion is accepted by most historians and rejected by some.