Rate of reaction?
How do they get the stuff to react fast enough to produce the neccessary volume of hydrogen? Even dropping bits of aluminium in acid doesnt do much more than bubble gently, due to aluminium's inconvenient oxide layer. You would need to optimise the surface area of the fuel pellets, which would mean costly machining of a resource which is going to be ultimately destroyed. You then need to manage the rate to ensure it continues to produce hydrogen fast enough to supply demand, but not so fast that an excess builds up, while having potential to supply more when needed, like when you need to climb a hill. Thats going to be some fiddly mechanics quite seperate from the engine, adding to the mass of the car.
Its also not that efficient in terms of mass of fuel to energy output. You carry 175kg of Al, and 350Kg of water to react with it, and produce 505.6Kg of Al(OH)3 and a measly 19.4kg of Hydrogen...
Whereas every gram of petrol carried is burned, at once releasing its stored energy, and reducing the mass of the car as the waste products are both gases, the hydrogen generator only release energy from the hydrogen produced. The Aluminium hydroxide is just additional, useless mass.
19.4kg of Hydrogen will release 7.8 x10^6 kJ of energy.
525Kg of Octane will release 2.5 x 10^7 kJ of energy, and you wont be carting round 95% of the original mass... Not really a contest.
Dont get me wrong, I think the idea has potential, but a portable hydrogen generator is not as convenient as it sounds, otherwise it would have already been adopted. The chemistry behind it is nothing new.