I don't have the answers, but some of the issues are:
Lubricity - care & feeding of fuel pumps
BTU's/lb. - range when weight limited
BTU's/gallon - range when tank volume limited
Cost/BTU - operating cost
There is also the very real environmental issue of BTU expenditure for production vs. BTU delivered to the tank. Farmers depending on the synfuels market don't like to discuss this, but ethanol/methanol production is pretty inefficient in waste heat.