Many fuels have stringent specifications. It seems to me that the process to burn plastic for making energy is much simpler than any process to turn plastic into a liquid fuel that can be used in engines. Both ways you accomplish the same result.
I do not want to burn unless I have to, as there will be need to look after emissions and would need stack treatment! What I see most claims for making fuel out of plastics is bogus as the energy requirements are never told! They also never seem to be willing to answer technical questions..I wonder why??? ;p
Most process that don't just burn it use pyrolysis. The problem is getting more heat out than you put in. This is most at the academic research stage at places like Georgia Tech.
Burnign can work better if you supply it with additional oxygen. Either way you still have to worry about the emissions.