AFAIK under elevated temperatures, it degrades nicely. At typical soil temperatures it slowly degrades into methane which is a greenhouse gas - not great for the environment... but it's still a hell of a lot better than plastic.
As bad as methane is, at least it has a relatively short life before it becomes Co2 and ultimately is absorbed by trees/etc and re-enters the cycle of life. Plastic on the other hand is really nasty toxin that often ends up in the ocean and causes long term damage.
The TLDR is methane needs to be managed, we have to make sure we don't produce too much. While plastic should just be illegal. We should never produce any plastic, at all, for any reason. It's going to take a long time but that's where we have to go.