The issue with hydrogen is that it has a fairly wide combustion range (meaning the ratio of fuel/air that can burn), I can't remember the numbers but it's several times greater than other common fuels. The other issue with hydrogen is that the combustion happens VERY fast... if you ignite gasoline vapor/air in an open 5-gallon jug, you have a nice rocket that'll fly 50 feet or so. If you ignite hydrogen/air in the same jug, you have permanent hearing damage and shards of plastic embedded in you.