Another aspect of this is that a JIT has to dynamically generate code to work: afaict, it's basically impossible to audit such a program to ensure that it only

Another aspect of this is that a JIT has to dynamically generate code to work: afaict, it's basically impossible to audit such a program to ensure that it only uses the public APIs it's supposed to use. Unless I'm wrong, the security of the Sandbox depends on such auditing being valid.