Is it true that bioplastics are a sustainable alternative to plastics since they degrade quickly in nature?

This claim is certainly false. Some of the materials called “bioplastics” are only biobased (made from renewable resources) but not biodegradable. Other materials called “bioplastics” are biodegradable but need very specific conditions to disappear quickly enough. However, these conditions are often not found in nature. If biodegradable plastics end up in the wrong place, they won’t biodegrade quickly and completely.