There are grey-goo outcomes if you are not careful with even something as constrained as an extrospection op code to existing language. There will be whole classes of not yet imagined grey goo opportunities lurking in a full TC language. You cant easily systematically defend against whole classes of such issues without intentional constrained language.
I figure the grey goo nightmare is an unspoken goal of Ethereum. Has Vitalik written on this elsewhere?