<p>Gneissic residual soils are typically heterogeneous due to the nature of such rocks and pedogenetic processes. This poses significant challenges for adequately defining representative geotechnical behavior parameters for these materials, especially in the case of linear works or risk management in hillside areas. This situation is critical in the Serra do Mar region, on the coast of Brazil, where landslides have caused hundreds of deaths in recent years, and some of the country’s most important cities and infrastructure elements are located. Although the heterogeneity and variability of gneissic soil shear strength have already been documented, few short databases with results reflect this variability. In addition, there are no references in which the impact of the variability inherent to these materials is assessed, which should be done in terms of probability and reliability analyses. In this sense, this work presents results of shear strength measurements and statistical parameters derived from them in the context of gneissic residual soils in an area at the southern end of the Serra do Mar, Brazil. Such data are used to demonstrate, through deterministic and probabilistic stability analyses of a slope with high FS and stable for many years, that the variability inherent to local materials results in low-reliability levels, indicating a higher risk of slope failure, even though the FS obtained in deterministic analyses is high. The performance of three different statistical approaches is also discussed. These findings have practical implications for slope stability analysis in similar geological contexts, providing guidelines for future research and engineering practices.</p>