The outcome of Terrain depends on you current location and probably a few other things too and according to The FF wiki there's a 40% chance of it using Shadow Flare in Southwest Saronia and Eureka, and a 2% chance in a bunch of other places.

I've never used the Geomancer for the same reason that I havent used the Archer and Bard jobs. They're a bit too niche to be useful when compaired to a Black mage or something similar.