I've demoed both, the Kastle FX 106 in 3-5" of light powder/crud. In the past, I've very much enjoyed the BMX 105 HP, but to me, the FX 106 finally felt like a freeride ski from Kastle: it not only flexed/rode/carved well on its edge, but it had the great 3D float effect of a modern ski. Just the turn shape and dynamic carve part of how it flexed and handled was coincidentally very similar to two skis I also tried: the Head Kore 93 and the K2 Mindbender 90. Of the three, in powder/crud I'd take the 106 probably, but overall, my favorite was the Kore 93, a ski I could enjoy owning. It's livelier, has a great dialed in feel, in soft snow at least - and is surprisingly good in at least some crud.
The FX 106 was to me a good start, an entry. But for me there are more dialed in freeride, soft snow skis of similar width out there from a number of brands, for me. My favorites are the K2 Pinnacle 105 (discontinued for this year, 19/20, in favor of the Mindbender 108, but still available) and the Faction C.T. 3.0 (also 108).
(The Mindbender 108, for me and a K2 associate I ski with, is apparently a bit unpredictably hooky at the tips: it threw him pretty badly at one point last season, really injuring his ankle. And he's a guy who normally would never get thrown. For me, I'd have to ski more defensively with that ski - at least at speed. The Pinnacle 105, tuned with a bit of increased base bevel at the tip fore the contact point, has no such problems. Bulletproof.)