Zbrush | When do I need to use GroupsLoops for Mesh Fusion

CTRL CLICK and DRAG down twice and Mesh fusion will do its thing. So far from my observation any geometries which is below 200k or Subdivision level 4, Mesh fusion will work without any problem, but any surfaces above the resolution of 200k or Subdivision 4, Groupsloops is required to tidy up the edges for Mesh fusion to work.