It's not the non-determinism that's a problem by itself - it's that the system is intended to be general, and you can't even enumerate ways it can be made to do something you don't want it to do, much less restrict it without compromising the features you want.

Or, put in a different way, it's the case where you want your users to be able to execute arbitrary SQL against your database, a case where that's a core feature - except, you also want it to magically not execute SQL that you or the users will, in the future, think shouldn't have been executed.