The honest trade offs
Does hard water ruin tankless heaters?
It does not ruin them, but it is the main thing that shortens their life and degrades performance, because scale forms on the heat exchanger. The answer is descaling on the manufacturer schedule, and in valley water that is not optional maintenance. Softening the incoming water is the other half of the answer.
Is it really endless hot water?
It is continuous hot water up to the unit flow capacity. Beyond that capacity, running two showers and a dishwasher at once, the temperature drops. That is why sizing to simultaneous demand matters more than any brochure figure.
Will it save me money?
No figure is published here, because a saving depends on your gas rates, your usage and what you are replacing, and any specific number would be a guess about your house. What is defensible: you stop keeping a tank of water hot around the clock, and you gain the floor space.
Is there a wait for hot water at the tap?
You still wait for hot water to travel the pipe run, the same as with a tank. Tankless removes the running out, not the distance.