Correct....the reason Ai-Servo is not guaranteed sharp is that it never stops focusing as long as the AF on is engaged, whether at the shutter button or on the camera back. If you're shooting motion...you're focused on maintaining your focus point on the subject. If, on the other hand, you're shooting static, with Ai Servo, you're really trying to use it like One-Shot AF...and but while you held the AF-on engaged, you may have moved off the intended subject point slightly. And there is the additional confounding factor that Ai Servo is apparently less light sensitive in practice than One-Shot. More sensitivity to light means more sensitivity to the point of intended focus.
Nevertheless, Ai Servo works exceedingly well across all of the newest Canon upper range, and is pretty darn good going back to the 7D, 1D Mk III and 1Ds III.
Reading across many, many photography forums, it becomes plainly evident that many, if not most focusing problems are more related to operator error in misusing or mis-understanding the focus system of their camera and setting it up correctly for the type of shooting being done.
Two things were clarified to me in the OP, the light sensitivity difference and the logical accuracy difference due to time to acquire focus between the two modes. I certainly am a bit more informed by this thread.