What are the limitations of using Selenium with Flex based UIs ?

What are the limitations of using Selenium with Flex based UIs ?
Solution: There is a relatively new open source project called FlexUISelenium located here, which allows Flex based UIs to be tested by Selenium. FlexUISelenium is an extension to the Selenium RC client driver that enables the Selenium RC client drivers to interact (and test) the Flex UI components and methods of the Flex application. FlexUISelenium works on the same lines as the Selenium Flash UI testing concept make calls to the ActionScript via JavaScript from Selenium.
Pros:
1)With FlexUISelenium the developers do not need to expose every method to be tested by using the ExernalInterface technique.
2)Just adding the SeleniumFlexAPI.swc library to the Flex application while building automatically exposes all methods to be called from external JavaScript.
Cons:

• The FlexUISelenium project is quiet new and has some issues as reported on the project home page.
• Its not production ready yet and hence not very reliable.
• The developers have to include an external library which is otherwise not required for production builds.
• How would you test Pop-up window using Selenium ?
• Solution: First steps would be to determine whether this is an alert dialog -created using JavaScript's alert() or a bona-fide pop up window probably using open(). Since they both are treated differently its important to understand what kind of test case it is and what exactly interviewee asking.