Burning methane also produces steam. Methane produces 891 kJ/mol, hydrogen 286 kJ/mol, methane has four hydrogen atoms that'd be 1144 kJ per what should the unit be in any case: Methane produces less heat per unit of produced water than hydrogen (the hydrogen first needs to get ripped off the carbon). Those ovens burn dryer than your current gas oven.

Never used steam when making pizza, they're not in there long enough for steam to make a difference. For bread it's indispensable to get a proper crust, though.

EDIT: Did I get moles right? It's been a while and I am no chemist.

Steam may be okay for the wheat crust, generally when baking bread steam is applied in the initial rise period, but is generally turned off at the end for a dry final bake. The cheese is another matter. Ideally the cheese has to do more than melt, it should develop a partly caramelized appearance on the top (slightly brownish in places). Whether that would happen with this kind of oven is unknown.