As a current Katana owner, I've wondered about these skis. I used to ski on the Theory, and when that was damaged I replaced it with the Katana. The Katana seems like a fine ski--perhaps more stable when flat, as you mention. And then people complained and they reintroduced the Theory as the Union.
Katana and Union have same basic construction and surface area. Now they have replaced the Katana with the Terrain? Perhaps a smoother turning Katana?
Seems like the Terrain and Union are very similar skis.