That's what I am worry about. Capacitor is charged by voltage and current deliver from DC source. AC does not charge capacitor. It goes straight through it. Lower the voltage means lower the current since that's what these devices are controlling. Current. Lower the current means lower the time it charges the capacitor or lower the time that current transfer throughout the circuit. That means, some parts of the circuit board will not get enough power supply at fast enough time. Capacitor will charge slowly and the time between charging and discharging will be big.
If you consider then RAM structure alone here, it requires 3 V for RAM to work efficiently. Even with 3 V, it requires pre-charged circuits in that microscopic structure because current cannot travel fast enough. Lowering this voltage is very inefficient. It may even cause MOSFETS to not work properly since the low gate voltage wouldn't fully open the circuit.
On the other hand, if you are talking about lowering a few milivolts, then it doesn't matter. It wouldn't affect as much as it would as lowering 5 V supply to let's say 2 V. Lowering the frequency will not cause any problem because that's just limiting the power consumption of the processor. In these devices, the most power drain is processor, video, data communication and backlights. So, lowering frequency of the processor, slowing down the data communication, turning off graphic intense apps and lowering backlights will use the device power way less and device will work efficiently. Lowering the voltage? It will cause damage to some parts of the device in long run.
Remember, these are nano circuits. Not only capacitor is important. Modern electronic does not work the way it did before. Characteristics changes as the size gets smaller.