No, failed work units simply are not reflected in the statistics.

The only way to track is keeping an eye on the 'Device Statistics' under 'Device Manager' in 'My Grid' which shows number of work units successfully returned and score. Came back from a 3 week absence and an UD agent that had not been able to return a WU due net failure for that period. Got the WU counted and points credited, so there is substantial leeway.

It would have been nice if the UD agent showed not only cumulative points and time, but also the work units so one would see it increment every time new work is fetched.... but that's a different story.