High initial electrochemical performance does not necessarily translate into durable energy storage. This critical review examines the capabilities and limitations of conventional characterisation techniques in assessing electrode degradation during prolonged operation of batteries and supercapacitors. Carbon-based, non-carbon, and biomass-derived electrode materials are discussed in relation to interfacial ageing, structural damage, heterogeneous ion transport, and loss of electrochemically accessible sites. Particular attention is given to distinguishing directly measured performance deterioration from diagnostic signatures and degradation mechanisms supported by complementary experimental evidence. A qualitative methodological framework is presented to guide the interpretation and integration of electrochemical, structural, morphological, and chemical characterisation results, while considering device-specific charge-storage mechanisms. The importance of monitoring cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance responses at different stages of electrode ageing is highlighted. Furthermore, the review discusses the limitations of conventional capacity and capacitance retention metrics and emphasises the value of time-resolved measurements and quantitative descriptions of degradation progression. These considerations provide a methodological perspective for improving the reliability of electrode durability assessment and supporting the development of more stable energy-storage materials.