I have no vested interest in anything here; I just wanted to share what I have found out about instant hot water heaters. My hot water heater is the Noritz NRC-661A-DV. Yours might be different but the issue with water hardness will be the same.
Instant hot water heaters have some advantages over tank designed systems primarily economically; because you only are heating the water you are actually using.
However, they do require maintenance due to the suspended solids in water coating the heating tubes, rendering them inefficient. The harder the water, the more maintenance will be required and probably service calls to flush and de-scale the heating tubes. The first week we were in our new house, I noticed that there was almost instant hot water to the shower located on the enjoining wall but, over the weeks, it seemed that the heat was taking longer and longer.
The link below will open a pdf file that has more about the problem of hard water and what I am doing about it. It includes links which should help. Hopefully, this information will help prevent you from being blind-sided with unexpected water heater issues and costs.