Nothing wrong with PEM fuel cells for vehicle applications, and many of the hydrogen storage issues are overblown.
The problem still is that making hydrogen is still inefficient. Electrolysis is inefficient, and so even if fed with free solar energy which has its own conversion efficiency problems. Reformation of liquid fuels makes a ton of sense, but start times and whatnot are challenges.
FCs dont have carnot efficiency issues, and the conversion efficiency of an FC can be roughly .7V/1.23V=57%, higher at lighter load. Far better than even the most advanced diesels, if the hydrogen production efficiencies can be kept high.
Cost, commercial adopters, and need for energy storage are some of the challenges that make it tough to adapt.