From what I remember, both types of mundane spawns were reduced during the invasion and the powerful rogue undead units didn't appear until after the invasion ended. It has been hinted on the D-list that this was a consequence of something achieved by the undead player during the invasion.
I will say that my shiny new adventurer was able to farm undead in a BT region for about a solid week alongside at least one other adventurer without running out just recently. From my last adventurer, I believe that's somewhat unusual, but I don't know for sure.
