Car hydrogen generators can help a bit with gas mileage but is it bad for the car?

Car hydrogen generators make a small amount of hydrogen to save on gas so they say, But when you mix air and hydrogen don't that make water? Or is it when its ignited it makes water? or both??
ether way this cant be good for a car? water or steam in the pistons cant be good, if there is any blow by it will get into the oil, and it cant be good for the pistons rings cat. o2 and muffler,,

Is this safe even if it does help on gas?