The only major caveat with hydrogen is that it is has quite a low energy density unless compressed to a ridiculously high psi and thus the need for an extremely strong tank.
I'm not sure that were at the stage yet that we can afford every Joe and Mary to be driving around with pressure vessels full of a combustable liquid on board
(At these pressures it is in liquid form).

More over, whilst hydrogen is the most abundant element in the universe it tends to arrive stuck to other things, requiring energy to separate it through a chemical process or electrolysis; as with the kit above. You won't get something for nothing or even for €184, and with the kit above even if their electrolytic reaction is 100% efficient (it won't be) you are still only converting petrol into electricity through your alternator, which in turn separates out the hydrogen in the water, which goes back into the engine. You're converting energy dense petroleum into low density hydrogen; zero sum gained.
Whilst the theory holds true that hydrogen can be retrieved from water, in real world scenarios, namely with retrofit kits like these, they are in the same category as Turbonators, fuel sharks or fuel line magnets; sudo scientific nonsense.

Spend the money on a good service and a bottle of diptane instead.