Directional drilling using Rotary Steerable Systems (RSS) enable accessing unconventional oil and gas reservoirs. However, this technology faces challenges such as unwanted borehole deviation, spiraling, and environmental uncertainties and disturbances. These challenges can lead to instability, suboptimal performance, and even potential costly failures during drilling operations. In this study, the Perneder-Detournay (PD) model is employed to represent the directional drilling borehole evolution process using nonlinear Delay Differential Equations (DDE) with distributed delays. Directional drilling environments inherently include complexities such as undesired vibrations, formation washouts, and uncertainties due to hard-to-model drag forces between the drillstring and the borehole walls. Unlike previous studies that primarily focused on quasi-constant disturbances, this research examines time-varying disturbances, providing a more realistic assessment of system performance. Additionally, this study investigates uncertainties in the lateral steering resistance, expanding beyond the traditional focus on Weight-On-Bit (WOB) uncertainties. A controller based on a spectral approach is developed to ensure system stability and optimize the transient response by guaranteeing that the rightmost closed‐loop pole lies within the complex left half‐plane, while real part is minimized. Thereby, the controller ensures accurate borehole tracking despite uncertainties and disturbances. Simulation results show that this control technique improves the drilling performance and prevents potential drilling failures in unconventional reservoirs, despite uncertainties and disturbances.