My understanding is that because hydrogen is so reactive, it tends to combust too soon. The heat from the inlet valve can make it explode too soon. Plus it burns so hot that it's easy to melt your pistons. As well as that, (Thus far) there's no way to make it effeciently. IE, for the amount of electricity needed to make the gas, it would be cheaper to just have an electric car.

Dont let that stop you though.