I have been thinking about the sandbox, and it has occurred to me that lambda functions should not be permitted for code running in the sandbox. Consider the following:

1. User installs a plugin which evaluates functions based on global variables.
2. A global variable is set in the sandbox with a lambda function.
3. User triggers that plugin somehow, which in turn executes the lambda function from the setting.

You can't define functions in the sandbox, so this isn't an issue with the :function command, but is with lambda functions.