The behavior of slender structures when subjected to seismic actions is often modeled as rigid rocking blocks. Unreinforced masonry structures, such as industrial chimneys and minarets, generally characterized by large aspect ratios, often fall into this category. However, simplified approaches like the two-dimensional rocking block model often neglect the influence of three-dimensional effects, which can lead to significant uncertainties in predicting the seismic response of such structures. This study investigates how this aspect influences the dynamic response of rocking systems. Two models are considered: (a) the two-dimensional rocking block model, which simplifies the problem by idealizing the structure as a rigid block, and (b) the three-dimensional rocking cylinder model, which provides a more refined representation by accounting for the full spatial nature of the system. While the latter allows for greater accuracy, it is also highly nonlinear and involves more complex formulations. To evaluate and compare these models, the seismic response of a case study involving the Dickson industrial masonry chimney in Montreal is analyzed under natural ground motions. The analysis focuses on identifying similarities and differences between the maximum rotations of the two models, with particular attention to the principal uncertainties associated with the widely used two-dimensional approach. The findings highlight the importance of considering both horizontal components of ground motion when using simplified two-dimensional rocking block models.