Have they gotten any better?
The last house I lived in had on demand heat, I cod get half a sink of hot water. That's it.
The Takagi on-demand I put in last fall can run up to 6.6 gpm at 140° for pretty much as long as you want. It doesn't like low GPMs though. It regulates the burner and when you get down around 1 to 1.5 gpm it can cause it to over heat and shut down the burner for about 5 to 10 seconds then fires back up. If it does that too many times it will trigger a fault and shut down totally until you clear the fault. It's a power vent exhaust so it can be exhausted up or out, which ever you chose. you can also choose to run an fresh air intake to the outside if you want, but they suggest gainst it in colder climates.