We have used script runner functions in issue filters successfully but have not been able to use them in workflow validators.
The objective is to prevent the closure of an issue if it has linked issues that are still open. For example issueFunction in linkedIssuesOf("status = open"). The validator returns a workflow exception indicating No such property: issueFunction. I suspect this may be due to missing import statements or just bad syntax but have not been able to find any examples of using the script runner issueFunction in a validator.