FX84s are much lighter in weight (evidently designed for back country) than most skis.
They bend longitudinally, easily, so I can get a nice controlled "scarved" short radius turn out of them with a minimum amount of skidding on New England hard snow. They are quite torsionally stiff, and damp in that Kastle way, so they hold an edge great on ice.
Because they bend easy, they're also good in bumps. In dense crud they readily bend when tipped into very short radius turns. Those two sheets of titanal are very thin. These are not MXs. Wood's in there too, along with the damping material.
It's been hard for me (135-140 lbs) to get rebound out of them, but I suspect if I were to move with quicker timing I could (aka pilot error). But then again these are not race skis, so I'm not sure.
I know one shop in NH that carries Kastles, but they don't have FXs.