Water will vary significantly in terms of corrosive and scaling potential in espresso machines. It isn’t possible to maintain a nearly neutral LI that is neither corrosive or scale forming. In steam boiler applications if any hardness from Magnesium or Calcium is present with bicarbonate then you’ll see scale forming as the minerals concentrate (assuming you start with a neutral or nearly neutral LI at the operating temp). Similarly, in a brew boiler, you may achieve a nearly neutral LI when it is up to temperature, but it will be corrosive when the machine is off and the water cools down to room temp. Whether it is a dual boiler or HX machine it isn’t practically possible to achieve this balance in both boilers (or boiler and HX) simultaneously, especially when most people will be turning the machine off at some point….a far greater risk of corrosion comes from chloride and sulfate presence (with sulfate-reducing bacteria) depending on boiler material, especially if scale forms.
The water recipes recommending hardness generally go with the idea that hardness influences extraction. Even tests involving gas chromatography are contradictory on this. As far as anecdotal reports re flavour are concerned, you will find they vary wildly. Some say magnesium produces a fruity cup, some say Calcium does, and some say coffee made from distilled water is best. My recipe uses Magnesium Bicarbonate and Sodium Bicarbonate, but it is not something I can recommend and flavour difference is minor compare to bicarbonate alone.