One of the most promising anode materials for high-energy sodium-ion batteries (SIBs) is hard carbon (HC); however, low initial Coulombic efficiency (ICE), unstable solid electrolyte interphase (SEI) formation, irreversible sodium loss, and sodium-plating risks at low potentials continue to limit its commercial implementation. The interfacial mechanisms controlling HC performance are critically examined in this paper, which also assesses methods for getting beyond these restrictions in realistic full-cell systems. The effects of surface imperfections, specific surface area, and open microporosity are highlighted in the link between HC microstructure, electrolyte breakdown, SEI composition, and sodium-ion transport. In addition, surface and artificial interphase engineering through atomic layer deposition, chemical vapour deposition, thermal-pyrolysis carbon coatings, and conductive protective layers is discussed for suppressing parasitic reactions and stabilising the interface; the mechanisms and operational triggers of sodium plating, such as high-rate charging, low temperature, and mass-transfer polarisation, are also examined; and finally, operando characterisation, scalable manufacturing, thermal management, and techno-economic considerations are identified as critical paths for converting HC-based SIBs into safe, long-lasting, and commercially viable high-energy cells