Your silver amalgam fillings are 50% mercury and must be removed and disposed of properly prior to burial or cremation. I'm not sure about the method mentioned in the article exactly, but it's illegal for dentists to dispose of amalgam in the trash because it contaminates the leachate of the landfill, so I think you'd have to get them removed prior to this too.

Not explicitly an argument against it, but I wonder if the plastic in humans at this point could be considered environmentally harmful. I suppose that if it's everywhere already, the decomposition of a body would add an insignificant amount more.