Engineering perspective: since the controller uses (as far as I know) Pulse Width Modulation (PWM) to control power output... the max torque/hp capability exists with 100V, not the 84V. The software may have the same top-speed limit (depending on software), and the 84V batteries with higher WH per cost will result in longer ranges. If the WH were the same for either 100V vs 84V, then the 100V would have longer range and more power. As for failures in controllers with 100V batteries, there is less "design margin" for things like reverse-EMF and heat dissipation, so yes failure rates should also be higher with the 100V battery in the same system. I'm also consdering this choice... reliability matters to me, as does performance... my ninebot one e+ isn't as fast as my Siberian Husky (his top is about 28mph).