The fuel cell is the engine. The problem is the fuel itself. Hydrogen atoms are the smallest atoms therefore they will leak out of any container made of a material made of larger atoms. To really make it safe you'd need to keep it in a box with lead walls 30cm or more thick, which massively increases the weight of the vehicle you're trying to propel. In open air or for a short period of time (like a day) it's not a very serious problem. But in an enclosed space, left for a week, dangerous concentrations of hydrogen are going to build up around the vehicle which can explode. There's also the issue that if you leave your car in a car park for 2 weeks to go on holiday all the hydrogen will leak out while you're away so you can't start it when you get back (as many people used to have problems with dead lead-acid batteries)