"Hard" water is water that is high in minerals, particularly calcium and magnesium, both of which plants need to survive, albeit in very small amounts.
Generally speaking, "hard" water shouldn't be toxic to plants, and could be beneficial, adding nutrients and mineral elements to the soil.
Hard water may be more of a nuisance to humans, mainly because it doesn't lather very well and it causes, as you wrote, scale buildup.

Here is a link that might be useful: Desert Gardens