It is really simple, overdraw means that pixels are drawn multiple times. And that is because material is translucent, does not hide any pixels behind it.
So non translucent drawn behind single layer/surface of translucent material is fine. However multiple translucent surfaces one behind another is quite bad. And you have some render mode (F3 in editor i think) to see that in view.
Best optimization for it is level layout and not abusing translucent surfaces.
TL/DR:
It is NUMBER of translucent surfaces one behind another that matters most.