Hydrogen is a really small atom...the smallest one actually. Natural gas is mostly Methane, a molecule with a molecular weight about 16x that of Hydrogen. Guess which will leak easier.
Hydrogen mixed with air burns about 600F hotter than Natural gas....easily deforming & melting parts designed for a natural gas flame.
On a volume basis, Hydrogen yields 325 Btu/cuft, while methane yields 1008 Btu/cuft. Your hydrogen would need to be stored and delivered at a much higher pressure through the same size pipes, and orifices.....again leaking more.
Creates 31,000 BTU. Requires 1/2 Gallons water / hour... oh and a 220 VAC 60A circuit.
Lets say it draws 44A @ 220 VAC = 9680 Watts = 32912 BTU spent making hydrogen for a return of about equal BTU's of heat from the flame. Oh..and you pay a sewage fee for the water this thing vaporizes.
I could see tree huggers taking home equity loans to upgrade to one of these, only so a coal burning plant has to be fired up for the 44A to power the hydrogen furnace.