A thousand times this.
Yeah, this is spelled out pretty clearly in the Doomhammer lore in Legion: Thrall's problems are entirely in his own mind.Thrall's powers suffered not because he cheated but because he forced the elements to kill Garrosh in his anger. Garrosh gained the favor of the elements in AU Draenor because he came with the warning of the impending Legion invasion they didn't want him to die. When Thrall forced the issue he used dark shamanism and his guilt over doing so has made him question himself.