There very few ions that can be added to boiler water without entering into the potential benefit vs. real disadvantage discussion. If you consider the actual espresso machine to be a consumable component of the coffee experience at 1 extreme then water chemistry can be driven by the final result in the cup alone. The other extreme is the espresso machine it to be protected at the expense of the final result. There are compromises that can be made but the distilled water with a buffer is one of several very effective compromises.

Coffee beans contain very little sodium, and much much more potassium. The same is true in wine. Wine makers add potassium bicarbonate for pH control because the potassium in the potassium bicarbonate is very small relative to the amount already there. They also use potassium because potassium bitartrate precipitates more. There is a salt substitute product that is 50% NaCl, 50% KCl. It tastes very much like NaCl. There is another salt substitute that is pure KCl, it tastes very different than NaCl. The same seems true here too, putting Na with K creates flavour changes.

Potassium bicarbonate is readily available, in part because of its use in wine making.

The very best thing about using a bicarbonate that does not have an insoluble carbonate is that the composition of the water is not always changing while one is making espresso. This, is of course, the result of not forming scale. (There is much more Ca and Mg in coffee beans with the usual amount of brewing water used for espresso, so using Ca and Mg free water has little effect on their concentration in the final product.)

I have mentioned it in earlier posts, but one of the weirdest things about the chemical composition of coffee is the fact that coffee contains higher Rb levels than almost any other consumed item. Rb is present along with all biological potassium, usually about 1/200 as much as K, but in come coffee it can be more like 1/90 as much. Some have claimed the Rb is essential to life. That has not been proven, however. Coffee has similar concentrations of Rb and Na.