If it is already scifi/fantasy and I want a different type of scifi/fantasy AU then the genre shift tag doesn't help. For example Sailor Moon is already scifi/fantasy. If I was interested in AUs like a high fantasy AU or a spaceship AU I could do AU - Scifi/Fantasy. AU - Genre Shift could be to any genre including mundane ones while the Scifi/Fantasy tag could lead to people who want to explore the already canon scifi/fantasy elements.