I have an on-demand electric unit in the shop. Hate it. As has been mentioned, they do not produce a fixed outlet water temperature under all conditions, just a temperature rise from inlet to outlet. So if you have a inlet water temperature that fluctuates, you have to size the heater for the coldest inlet. Then there is the flowrate. Unless you get one of the larger ones, the common smaller ones produce about 1 gpm of heated water. Then the maintenance, if you have hard water, buy stock in a vinegar company because you'll be using a lot to descale the heating elements. This is ignoring the current draw that you'll put on your service entrance, for mine if I wanted a good supply of hot water, I'd ended up with 3-40 amp 240 volt breakers.

If I had it all over to do again, I'd gotten one of the conventional small trailer house water heaters and called it good.