my main interest is seeing this type of technology introduced into transportation.
In addition to natural gas, they can digest liquid fuels and are much more efficient than the internal combusion engine. I want a full sized 4 wheel drive pickup with towing power that gets 30 mpg.
The design problem for that scenario would be driven by the surface area required to produce the necessary amount of power. That, in turn, translates into volume and mass for the fuel cell. How big and how heavy would the fuel cell have to be, to generate enough power to manage the load you suggest?
Also, you would need to factor in things like requirements for acceleration, different terrain, temperature ranges, and all that. I think those sorts of factors couldn't be dealt with easily by the fuel cells themselves, and would therefore translate into requirements for good-sized batteries, which increases volume and weight requirements.
It may be that, above a certain sized vehicel, the energy density of gasoline or diesel is better suited to direct use in an internal combustion engine, than it is for use in transportation.