A critical review on nano-enhanced phase change materials (NePCMs) is presented, underscoring the achievements, inconsistencies in reported data, and challenges within the field. The disparate effects of nanoparticles on the modulation of latent heat of phase change materials (PCMs) are comprehensively evaluated. The review delves into the thermal capacity and complex viscosity variations in NePCMs, evidenced by an in-depth mechanistic analysis. Additionally, the review summarizes the current research on the thermal conductivity of NePCMs across solid and liquid states, providing thorough discussions on the underlying principles and mechanisms. It is found that the uniform dispersion and long-term stability of nanoparticles within PCMs are pivotal for consistent thermal performance. While the thermal capacity of NePCMs is generally reduced due to the addition of nanoparticles, although few studies have observed an increase in both latent heat and specific thermal capacity. The incorporation of nanoparticles typically increases the viscosity of NePCMs, which remains intricate due to the variability in nanoparticle characteristics and potential aggregation. Nanoparticles have demonstrated potential in enhancing the thermal conductivity of NePCMs, yet inconsistencies in the mechanisms of Brownian motion and the formation of semi-solid layers are significant concerns. The review also presents the challenges and future research directions in NePCMs research.