[Syonyk]

First, there's a lot of energy in the chemical bonds, so depending on how you're incinerating it, you can get some useful energy out of it. The Hefty Energy Bag program does this - they did a lifecycle analysis on various plastic "end of life" paths from "landfill" to "advanced plastic thermal decomposition" to "burn it in a cement kiln" - with the last one working by far, the best. At last per their analysis. If you're offsetting coal use, which is what would otherwise be burned in the cement kiln (I believe natural gas and hydrogen don't emit enough radiation because of their lack of carbons to be as useful), great.

Second, done properly (insert a lot of observations about combustion temperature here), it ends up as nothing worse than CO2, nitrogen, water, etc at the exhaust stack. Given how horribly bioreactive plastics tend to be, and their tendency to erode into microplastics given half an opportunity, this is roughly the "Flare the methane to CO2 because it's far less bad" end of plastic compared to burying it, which, at some point in the future, stands good odds of being uncovered - perhaps by a group that doesn't understand just how nasty the stuff really is.

If your takeaway is "There don't sound like any great ways to deal with plastic," good. Because there aren't.