Lightning remains a major threat to the safe and reliable operation of transmission lines. To comprehensively evaluate the accuracy of existing lightning flashover criteria, this study statistically analyzes typical lightning current characteristics, including waveform and polarity, based on data from artificially triggered lightning experiments and lightning location system monitoring. The statistical results indicate a significant increase in the proportion of positive lightning in nature, and the presence of subsequent lightning currents with short tail durations under multiple strikes. Then, PSCAD simulations are conducted to compare the overvoltages generated at both ends of insulator strings under different lightning current waveforms and strike types, including lightning shielding failure and back-flashover. A review of existing flashover criteria is also presented to highlight their applicability and limitations. Furthermore, artificial-intelligence-based methods for predicting flashover characteristics are comparatively analyzed, focusing on feature extraction, model performance, and generalization capability. Finally, the study summarizes the key issues in current research on flashover criteria and outlines the main directions for future studies aimed at accurately assessing the lightning protection performance of transmission lines.