Fuel Cell is extremely expensive at the moment... You need a constant supply of hydrogen (which you can´t usually find in your local gas station). You can then produce it but is gonna cost you quite a lot in the long run, you´d better have a huge fleet plus a good H&S implementation to avoid potential accidents, which in case of hydrogen can be fatal.

Yes, it´s supposed to be clean, probably the cleanest. Yes, if you have supply is quick to refill... but with all the caveats above. I´d definitely not go for this technology for the moment.