Had this installed on an exterior wall of a 1800 sq ft townhome 3/3 we lived in while working on another house we live in now. The townhome is a rental. Installed in 2011 and still going strong. No exhaust issues since it is external to the dwelling.
Rheem RTG-95XN Tankless Outdoor Natural Gas Water Heater
The newer version is this I think(bought it from these guys as well):
https://www.homeperfect.com/rheem-rtg-95xln-9-5-gpm-outdoor-tankless-low-nox-water-heater-for-3-bathroom-homes.html
We drip the lines closest to it, on those few days in DFW that are sub 32 degrees, so water flows even in freezing weather. We have Pex water lines all on that exterior wall. That was easy to do when we installed it because that wall was gutted. There are foam covers for the lines but otherwise it has worked fine.
Electric tankless work fine but are the least efficient in terms of energy use or that was the case back in 2011. Sometimes you pay more to have the small ones under the sink at the furthest point and they are electric, still way inefficient, but you don't wait for hot water.
Another house we have was an attic water heater replacement, nogo on the tankless due to the expense of rerouting larger gas pipe, more exhaust needs, etc. So we just went back with a 50gal NG model and the plumber added a better drip pan to bring it up to code.