If you throw a pilot into a scenario where he or she will have to repeatedly perform an action that is not part of their day-to-day experience (such as manually trimming a FBW Airbus), then it adds a risk factor because they may not get it right.
They may or may not get it right - no one knows for sure. We may suspect there is a risk, but in absence of real data supporting one point of view or the other, they are both just speculation.
AF447 is a real-life example of a situation when Alternate law did not save the day, and possibly even contributed to the events that led to the crash. Are there real-life examples of the contrary: situations when the plane suffered malfunctions and reverted to the alternate law, and in that condition successfully brought back to a runway in one piece?
EDIT: never mind this post, I now realize that the same situation happened each time there was unreliable airspeed.