A potentially beneficial conflict management tool, post-conflict power-sharing is often marred by political crises and instability. We explore the link between power-sharing ‘adoptability’ and functionality. Focusing on Iraq, we demonstrate that weak adoptability (when one of more groups have grave reservations about the settlement) harms power-sharing in several important ways. First, weak adoptability by the Sunni community led to their exclusion and mobilisation against the state. Second, weak adoptability provided political space for Shiites to dominate in politics, undermining the very basis of post-conflict power-sharing. Third, weak adoptability further compounded constitutional contestation, leading to the Kurdish independence referendum in 2017.