You might run into what I ran into 🙂
The help of the tool states as the first "pre-requisit":
Classify LAS Ground
Only the last return of LAS points with class code values of 0, 1, or 2 will be considered for reclassification as ground. If your LAS files use different class code values to represent unclassified or ground measurements, consider using the Change LAS Class Codes tool to reassign them accordingly. The classification process will also ignore points that are assigned with either the overlap or noise classification flag.
So in fact if your data is REALLY unclassified - ArcGIS will not classify it. You (still) need additional tools.