If the plastic to be burned substitutes for coal or oil, it is carbon neutral. Isn't that what the Scandinavian countries do with their trash as an alternative to landfilling it?
Not burning the plastic risks its turning into microplastics, which will tend to interfere with the physiology of all plants and animals.
It's not carbon neutral, it still adds to the problem. We need to replace our carbon emitting power generation with renewable energy, not burn our trash to keep emitting the same. And trash can just be buried, it doesn't need to be burned.
There's a lot of talk generally of running carbon sequestration technologies and how important that will be. Burying plastic waste is exactly doing that, without spending the extra power to actually extract the carbon from the air.
Not burning the plastic risks its turning into microplastics, which will tend to interfere with the physiology of all plants and animals.