Still very raw implementation of expression function.
There is no support for jira-way jql short dates like: 1w, 2d, 3wd etc...
There is no support for JQL function calls, like now(), currentUser() and others
GroovyShell runs is not safe (maybe there is possibility for some security vulnerabilities here)
Right now all script execution fails will cause RuntimeError in getQuery function. This is not good way to deal with them because most of the user input errors (like adding or comparing not comparable types) can be found only during script execution. On the other hand jira api forces all the jql-functions to show input errors on validate() stage.
Still very raw implementation of expression function.