My personal reckon is that hydrogen fuel cells will become a big thing in larger vehicles because battery tech is hard to scale up to high production volumes and capacities, and the elements needed to manufacture (ie Lithium in large amounts) is hard to find. Imagine how many cells an electric truck or bus will need compared to a bicycle. Hydrogen, well, that comes from air or water and all you need for bigger vehicles is a bigger tank.
But what about in ebikes? For most uses, the current battery tech works OK and the scale of production required is relatively small. But how about fleet bikes such as delivery cargo bikes? Maybe hydrogen is relevant.
FWIW a hydrogen fuel cell is effectively a battery – the hydrogen is extracted typically from water which requires energy, it needs to be compressed which requires energy, and transported to where it can be used. In this way it is not that different to LPG except that LPG is a fossil fuel and creates CO2 emissions. The fuel cell simply converts the hydrogen to electricity. It is said that it isn’t a particularly efficient way of storing and transporting energy, but also has some advantages if that energy is got cheaply and sustainably in the first place (eg solar)