I have a 36 kw unit made by Eco Smart. It's in my own house I'm renovating. It's perfect since when I first installed it, I wasnt staying there full time so would be in and out randomly when I had time to work on the house. Didn't have to keep a tank unit on all the time and still had hot water as soon as I got to the house after not being there in days/weeks.
4 40 amp breakers, well water, and the elements turn on sequentially one at a time based on demand.
200 amp service, small 900 sq foot house. Haven't had any issue what so ever, lights don't flicker.
No access to gas on the property, everything in the house is electric, including baseboard heat. Installing a multi zone mini split currently. So far, I'm a fan of the on demand heaters. Time will tell. It's been the only source of hot water there for about 2 years. No breaker or bus issues what so ever.