The standard problems with on demand water heaters still apply. With gas, you need to vent it outside and it also requires 3/4 inch gas lines so you have the additional costs of adding gas lines and relocating it as most gas water heaters just use a 1/2 inch line. Plus the usual problems of them only have a 5 year warranty, you can get a regular gas tank water heater with warranties up to 12 years but if you change the anode rod, it could last 20 years. Plus because there are many manufacturers, when on demand heaters breaks, you can't just get a part easily and it's hard to find someone who knows how to repair them. Gas water heaters tanks are pretty basic, there's the burner assembly, the thermostat and the thermocouple, those are the three basic items that go and there's only a few big makers of them so it's not that hard to find parts. Easy swap out if they go bad. Also if you look at the costs involved, it can take years to make your money back and as a landlord, the tenants pay for the gas so any gas savings doesn't apply. I still see water heaters in most homes. You only see the on demand stuff in new construction and in the expensive homes.