Disclaimer: the below article is a simplification of the processes of and problems with incineration. For more significant and robust analysis of the concerns with incineration, see GAIA's fully-referenced reports on incineration, pyrolysis, and gasification.
Whether the question is traditional incineration, gasification (often plasma arc), or pyrolysis, "transforming" plastic into energy is a deeply problematic process, for a number of reasons:
1. Burning plastic does nothing to prevent the global problem of plastic pollution.
Relying on any technology that purports to either convert or burn plastic does not change the culture of disposability and waste. Not only that, expensive incineration or waste-to-energy technologies create a demand for plastic garbage, locking us into a perpetual system of creating waste. In order for these technologies to be worth their investment costs, they assume a continuous feedstock of garbage to run through the mechanisms.
2. Taxpayers fund the construction of incinerators and waste-to-energy systems, and they don't work.
No community has so far gotten an efficient return of fuel or energy at the appropriate scale from any of these technologies. All of them are multi-million dollar enterprises which typically consume more energy than they produce. These technologies do not prevent pollution, and they require expensive specifications that communities frequently don't have the resources to fund.
3. The specifications for the systems that drive these "waste-to-energy", "waste-to-fuel", incineration, gasification, and pyrolysis systems are not transparent or available for public review.
Companies claim proprietary protection over divulging the core functions of their systems, preventing researches and communities from knowing their full environmental or health impacts before they're built.
4. Any systems that landfill or process garbage harm fenceline communities.
Communities of color, especially those that are low-income, continue to be disproportionately affected by landfills, "waste-to-energy" systems, and incinerators. These facilities consistently are built and expanded near communities of color, exposing vulnerable people to the costs of our planet's consumption. No community should be sacrificed or harmed for the sake of someone else's convenience or consumption.
Too many questions surround these technologies to deem them a safe or wise investment for communities.