Just my $.02
Hydrogen generators vary quite a bit in efficiency due to design. Dry cells tend to be the most efficient atm, while the rest are usually way way behind in efficiency and prove the notion that its snake oil. A well designed dry cell will usually put out about 1 Lpm per minute per 10 amps while the others struggle to reach even half that.
When you are driving the alt is running no matter what so there is waste energy available as I see it, sure pulling and extra 25 amps from a 100 amp alt for a hho gen may create load, though many have had good results on other vehicles. Hydrogen mixed with o2 is very combustible and when added to fuel and air, well most view it as a positive thing making for more complete ignition. Cars and trucks that use 02 sensors and computers generally compensate for a lean condition due to the better burn and compensate with more gas, in effect lowering mpg's. Most folks who have success use a dry cell, and either electronics or chip to out smart the computer, an egt gauge, and pwm.
Most folks use lye to promote electrolysis, but there are better options that are safer for aluminum etc. I am not willing to write this off as snake oil just for the simple fact that this is where fuel cell technology comes from, maybe it's not mature yet (baby steps).