True, but that's part of the way it works, same could be said for dropped MMM ball in the back of a mineral line, or Nydus Worm. You have to learn to be perceptive and counter. Anything in a large number could be considered overpowered. I personally think MMM is overpowered in general for a T1 unit.
If an enemy can attack my voids before they are charged, the voids will more than likely lose. The mistake most people make is trying to attack a group of fully charged rays by sending in a small number (or one at a time) of units. It won't work. You need either a considerably larger size force, or wait until they run out of things to shoot so that they have to recharge, then counter before they do. In the meantime, get your workers out and build a new base.
Also - Rays in large numbers have a hard time charging fighting against only units because the unit dies before they can get charged. Use this to your advantage too. Typically if I'm sneaking up on a mineral line, I will shoot a gas building first to charge up in case the person runs his units at me.