The successful installations have a very small reserve tank to provide preheated water for the first few seconds before freshly heated water kicks in from the unit, and there’s only one in the house. That’s the economical implementation. Putting one at every hot water outlet is more of a luxury thing and won’t save a dime, probably costs more but instant unlimited hot water does have an appeal, especially with a house full of people taking showers and such. No more running out or waiting.
I’m sure they’ve improved since I last checked them out (10 years ago.) I’ll look into it again.