I built a house about 2 years ago which had 2 of the largest electric tankless units made. When they operated, the voltage drop caused all sorts of electrical problems. They didn't operate as desired or expected. Repeated calls to tech support, swapping out the units, nothing worked until they were removed and a standard 50 gallon HWH was installed. No problems reported since it went in.
Those two electric on demands are setting in my garage. I can't give them away.