With a group drop, you
1. Effectively sever master instances from their masters, so the assembly will get out-of-date if you update the masters
2. A group-drop needs to be ungrouped, which results in an alert message that is scary for novice users.
To avoid point #2, you just need to make a simple Macro who gets called by the eventdrop like:
Sub degrouper(shpObj As Visio.Shape)
Application.AlertResponse = 1
shpObj.Ungroup
Application.AlertResponse = 0
End Sub