Medicinal plants contain chemically diverse mixtures of phenolic acids, flavonoids, tannins, terpenoids, alkaloids, vitamins and other metabolites that may participate in redox reactions. Their antioxidant potential has attracted interest in pharmacognosy, food science, agriculture and natural-product research. However, the measured activity of a plant extract is not an intrinsic property independent of method: it depends on species identity, genotype, plant organ, developmental stage, environmental conditions, drying and storage, solvent composition, extraction energy and time, assay chemistry, and the way results are expressed. This review critically examines the relationship between phytochemical complexity, extraction design and antioxidant assessment. Conventional solvent extraction is compared with ultrasound-assisted, microwave-assisted, pressurized-liquid and supercritical-fluid approaches, with attention to yield, selectivity, thermal sensitivity, solvent burden and scale-up. Common assays—including DPPH, ABTS, FRAP, CUPRAC and ORAC—are discussed in terms of reaction mechanisms, analytical strengths and limitations. Total phenolic and flavonoid assays are treated as operational measurements rather than compound-specific determinations. Chromatographic and spectrometric tools, especially HPLC-DAD and LC-MS/MS, are considered necessary for chemical characterization and interpretation of complex extracts. A central conclusion is that no single extraction method or antioxidant assay can provide a complete ranking of medicinal plants. More reliable studies combine validated chemical profiling, complementary antioxidant assays, transparent reporting, appropriate controls and, where relevant, cell-based or in vivo evidence. The review proposes a practical framework for improving reproducibility and avoiding unsupported translation of in vitro antioxidant results into therapeutic claims.