As part of the community stress‐drop validation study, we evaluate the uncertainties of seismic moment and corner frequency for earthquakes of the 2019 Ridgecrest sequence. Source spectra were obtained in the companion article by applying the spectral decomposition approach with alternative processing and model assumptions. The objective of the present study is twofold: first, to quantify the impact of different assumptions on the source parameters; and second, to use the distribution of values obtained with different assumptions to estimate an epistemic contribution to the uncertainties. Regarding the first objective, we find that the choice of the attenuation model has a strong impact on results: by introducing a depth‐dependent attenuation model, estimates of events shallower than 6 km increase of about 10%. Also, the duration of the window used to compute the Fourier spectra show an impact on : the average ratio between the estimates for 20 s duration to those for 5 s decreases from 1.1 for to 0.66 for . For the second objective, we use a mixed‐effect regression to partition the intraevent variability into duration, propagation, and site contributions. The standard deviation of the intraevent residuals for is 0.0635, corresponding to a corner frequency ratio . When the intraevent variability is compared to uncertainties on , we observe that is generally larger than the 95% confidence interval of , suggesting that the uncertainty of the source parameters provided by the fitting procedure might underestimate the model‐related (epistemic) uncertainty. Finally, although we observe an increase of with regardless of the model assumptions, the increase of with depth depends on the assumptions, and no significant trends are detected when depth‐dependent attenuation and velocity values are considered.