I'm testing my app with iOS 8 and there are several issues. The scanning is slower and after different targets OpenGL stop to draw the background, and also the the lines of Vuforia Clouds stop working.
The Image tracking continuos to work (I can log the image that it finds..) and the app is usable in other parts (UI controls, etc..).
Other times, using the method pauseAR the app freeze itself without a crash or logs o something.
Have you experienced similar issues with iOS 8?