Flammability of composite materials represents one of the major bottlenecks in Aviation industry. PVC as a representative charring material and PMMA as a representative non-charring material were considered for this study. The predictive capability of pyrolysis models developed via a calibrated semi-empirical approach is examined through a series of numerical experiments. These configurations feature conditions that are significantly different from the reference conditions used in the model calibration phase. It is found that the domain of validity of semi-empirical pyrolysis models is limited to the conditions that were used during model calibration and that extrapolation to non-calibrated conditions results in a significant error. To further examine the uncertainty in pyrolysis modeling, the effect of unsteady incident radiant heat flux was considered. Results indicate that in a time-average sense, the oscillation in the incoming heat flux does not affect the rate of formation of flammable vapor. However, the timeline analysis of charring and non-charring materials response shows quite different trends.