It's been a long time since I've read up on this, so things may have evolved a bit. That said, I believe the challenge lies in a safe way to store the hydrogen while on the vehicle. H2 gas is incredibly explosive, and requires a heavy/strong/expensive tank for safety. This added weight, complexity, and issues with overall safety counters a lot of the benefits. A method for storing/saturating hydrogen into a solid state medium has been developed, and is presumably much safer than hauling around dangerous H2 gas. However, the catch here is that it takes some crazy high temperature (~900 degF) to release the hydrogen from the media for consumption. There are also reformers that can split H2 gas out of natural gas, but that still kinda puts you back where you started with the need to consume a fossil fuel. However, the reformer to fuel cell would be a pretty clean way to utilize natural gas, vs burning it in a combustion engine.