I dont think neurons have a reliability problem. Most synapses use ligand-gated channels, and neuronal homeostasis makes sure neurons do fire within their physiological range. Stochastic channel opening has not been found to have conclusive effects in reliability. Cortical neurons live near the surface and the brain solves the surface area problem by creating folds - the bulk of the brain volume is axons which form the white matter. If there's a tradeoff that evolution had to make, i speculate it would be between head size and vaginal opening size.
