Oh boy... something like that in the US... here in Europe those beasts are wired across the three phases and still require a 3x35A 400V feed (that's a standard service for a single family building here...)
They only have one single advantage... they provide instant hot water where no gas is available. That's the only reason they were invented.
Tankless gas water heaters have been de facto standard here for ages. Apartments built around 1900 and later (the big majority of Vienna's houses) didn't have hot water originally, or if they did it was already tankless gas water heaters. They were invented in 1885 I think. Over the years functionality to add hot water central heating to small wall-mount combi units was added (caught on around the 1970ies). Nowadays you can see such a tankless water heater/central heating combi boiler in almost every renovated apartment and smaller single family home (those combi units serve a floor area of roughly 2000 square feet for heating).
