Transient-skyrmion mediated voltage-controlled magnetic anisotropy (VCMA) switching is a fast and energy efficient way to magnetization reversal, but its reliability under realistic material disorder and thermal fluctuations remain unclear. In this work, we use stochastic micromagnetic simulations of a 100 nm perpendicular magnetic tunnel junction to study switching reliability under grain-scale variations in Dzyaloshinskii-Moriya interaction, perpendicular magnetic anisotropy, easy-axis orientation, and intergrain exchange coupling. We find that switching remains robust under moderate disorder, with 100% switching observed in 10^4 stochastic simulations after pulse width optimization at both 300 K and 350 K. Increasing the temperature from 300 to 350 K shifts the optimum VCMA pulse width from ~475 to ~450 ps, indicating that an apparent reduction in switching probability at a fixed pulse duration can be primarily due to pulse detuning. In contrast, strong easy-axis disorder reduces the maximum switching probability even after pulse-width optimization, indicating degradation of the switching dynamics. We further show that, under strong easy-axis disorder a transient skyrmion still forms while the subsequent post-pulse dynamics fails to complete the magnetization reversal. Our study therefore establishes the distinction between recoverable reliability due to timing shifts and permanent disorder-induced degradation that simply cannot be recovered by pulse re-tuning. Finally, for no disorders but at room temperature, 100% switching is observed with 10^6 stochastic simulations with correct pulse shaping.