Buddhism and Jainism disagree with each other on a great deal, but both hold that a soul's moral choices causally shape its future, that karma, understood as the working-out of intentional action, is real and consequential. Ajivika niyati denies this at the root. Where karma doctrine makes the moral life meaningful precisely because effort changes outcomes, Ajivika determinism holds that every soul passes through a fixed, predetermined sequence of states across an immense span of transmigration regardless of what it does, so that what looks like virtuous or wicked action is simply the unfolding of a course already fixed, not a cause of anything that follows. Liberation comes, on this view, only when the fixed sequence itself finally runs its course, not through discipline, merit or insight.
This is why Buddhist and Jain sources reserve some of their sharpest polemic for Ajivika teaching specifically, sharper in tone than what they direct at each other. A rival that denies effort matters at all is a more fundamental threat to a religious system built on cultivated discipline than a rival that merely disagrees about which disciplines work. The dispute is also why niyati remains the single doctrine historians can state with real confidence about Ajivika belief: it is the point on which every surviving source, hostile as each one is, consistently agrees, and it is precisely the doctrine that made Ajivika teaching legible, and objectionable, to the karma-based traditions that eventually outlasted it.