A commonly used approach to simulate ion thruster plumes is a hybrid particle-in-cell (PIC) with the Boltzmann relation to model electrons. The limitations of such an approach are investigated by benchmarking hybrid PIC against fully kinetic PIC simulations using the real Xethorn to electron mass ratio (m(i)/m(e) = 241073). The results show that both full PIC and hybrid PIC lead to similar plume density structures but very different plume potentials. If one could ignore the ion beam neutralization region near the thruster exit, the main physics missing from a hybrid PIC plume model is that from the anisotropic electron characteristics. Including the beam neutralization region leads to further changes in electron temperature and plume potential. Compared to full PIC simulations, the hybrid PIC model results in an overestimation of the plume potential by about 30%-40% in the transverse direction for the simulations considered. Since the electrons in the plume are nonequilibrium and anisotropic, simply replacing the Boltzmann relation with the more general polytropic thermodynamic relation for electrons in a hybrid PIC plume model will not lead to a significant improvement in accuracy. Published under license by AIP Publishing.