Fluid viscous dampers are dissipation devices widely employed for seismic vibrations control. Despite the performance of systems equipped with viscous dampers has been largely investigated, the majority of the studies have been carried out through deterministic approaches, neglecting the response dispersion due to any source of uncertainty. This work aims at analyzing the effects of dampers properties variability, arising from the tolerance allowed in quality control and production tests, on the seismic response, evaluated by monitoring the most relevant Engineering Demand Parameters (EDPs). To this aim, a preliminary study is carried out to relate the variability of the constitutive damper characteristics to the tolerance limit allowed in control and production tests. Then, the seismic response sensitivity is studied by evaluating the influence of the allowed variability of the constitutive damper properties on the response hazard curves, providing the exceedance probability per year of EDPs. A set of linear systems with different dynamic properties and with both linear and nonlinear dampers are considered, and Subset Simulation (SS) is employed together with the Markov Chain Monte Carlo method to achieve a good estimate of small exceedance probabilities.