Hi Aneez,
In general, automation of WPF applications is not harder that automation of Windows native applications. Even more - as .Net applications are open for TestComplete, you can always access applications UI controls via their native methods even if the given control is not explicitly supported by TestComplete.
What makes automation of WPF applications harder, is that WPF graphics system places a huge amount of intermediate controls between, for example, the form and a text box on it. Though TestComplete has means to hide the unnecessary controls to ease the GUI hierarchy.
Developers can easily make things easier or harder for you, as a test automation, depending on the approach they use during development, but this is applicable to any application and is not WPF-specific.