On some devices, the VideoOutput component plugged to a Camera does not render the camera preview. The problem has already been reported several times for HTC devices: QTBUG-37525, QTBUG-40650.
The problem is that such devices have their aspect ratio of capture resolution slightly different from their aspect ratio of camera preview size. On my device, default resolution for capture is 2592x1952 (aspect ratio=1.328) whereas the default video preview size is 640x480 (aspect ratio=1.333). This causes QAndroidCameraSession::adjustViewfinderSize to never set the video size of the video output which, as a consequence, never renders anything.