The axioms that Buddhists use to define and delimit the theory of karma mostly derive from of an ideological program rather than resulting from careful study of nature. These axioms force Buddhists into incoherent or self-contradictory positions on karma that can only be addressed by ad hoc extensions and black-box processes. Those that were traditionally added, brought new metaphysical problems and beyond a certain point these are not addressed by the Buddhist tradition.
Simplistic models of karma break down when we add real-world complexity. What holds for one karma does not hold for two, let alone for a realistic number. This is not just poor abstract philosophy. People base their actions and their life choices on these ideas. If the argument presented here is correct, then karma is a poor basis for decision making because it doesn't make sense and doesn't explain how morality works. Furthermore karma is at the heart of Buddhism. As I have shown in previous essays, where there was a conflict between karma and the highly esteemed idea of dependent-arising (pratītyasamutpāda) it was always the latter that was altered to preserve the functionality of karma. Karma is primary. Without karma Buddhism unravels."