Video enhancement is generally more effective on a clean video source. Ideally, users should import a video in its natively-captured format - that is, the file as it was transferred right from the camera or sensor.
A video that has been transcoded (i.e., had its format changed from one format to another, e.g., from FLV to MOV) loses many of its high-frequency details and is more difficult to enhance. A video that has had its frame rate changed (e.g., a video that has had its original frame rate of 7 frames per second changed to 15 frames per second by frame-rate doubling) is also difficult to enhance since duplicate video frames can cause problems with frame analysis.
Forensic can still produce impressive results with videos that have been transcoded or have had their frame rates modified, but enhancing a clean source will typically produce optimum results.