The challenges faced in Handling Ajax Call in Selenium Webdriver are
Using "pause" command for handling Ajax call is not completely reliable. Long pause time makes the test unacceptably slow and increases the testing time. Instead, "waitforcondition" will be more helpful in testing Ajax applications.
It is difficult to assess the risk associated with particular Ajax applications
Given full freedom to developers to modify Ajax application makes the testing process challenging
Creating automated test request may be difficult for testing tools as such AJAX application often use different encoding or serialization technique to submit POST data.