I know something of this concept, having had three on-demand water heaters in campers.
The Unimog came with a German unit that worked quite well. I liked it so well that when I did the Sprinter, I got a full-fledged marine unit that works superbly. Unfortunately, it costs $1500 plus installation.
After about 13 years of use, a pinhole corrosion leak developed in the Mog's unit and, since more would be coming, I scrapped it for a $300 Asian unit off eBay. It got installed just before I stopped using the truck, so I've no long term opinion.
Anyway, I love these things compared to the alternatives. However, you have to understand that unit size and inlet water temperature have everything to do with your satisfaction. If you get a large, expensive unit, such as I have in the Sprinter, the unit will provide normal shower flows of hot water with near-freezing inlet water. But a littlest units may struggle to provide a 20 degree temperature rise when run at full flow. Getting 40 degree water up to 100 degrees will be diffficult with anything more than a trickle of water.
The issue of size and venting will be a considerable factor, since the units are wall-mounted. Any unit that will meet normal needs at normal flows is likely to be bigger than you'd ideally want to put on your wall.
One last point (AND THIS IS INFORMATION, NOT ADVICE; YOU'RE RESPONSIBLE FOR YOUR OWN SAFETY) is that a year or two ago, when I bought the replacement for the Mog, the smallest of these heaters were advetised as "no vent needed." The idea was that they burned little enough gas and ran for short enough periods of time--and they may have had oxygen depletion sensors; can't remember--that the exhaust could go into the air. Clearly, some protective agency disagreed (and there used to be ten times as many for sale on eBay as there are now), but at one point, the manufacturer figured that going without a vent wasn't totally absurd. (FWIW, I sort of agree; you'll only be running the unit when you're awake, more likely to be in a ventilated space, and pretty likely do something when you note that your the air is getting bad.)