A freestyle deck will handle all mountain riding just fine with just some limitaions. If it is a really soft street deck, it will chatter and become very unstable at high speeds if the runs are at all bumpy or tracked out. If you want a freestyle deck that also will handle decently all mountian, the key is the stiffness rating. A mis stiff deck is going to be the ticket. Something like the Atomic Hatcet or the Rome Agent are really ideal boards for this "multi tasking" a noodle like a Ride Kink or the Rome Machine will have more limitations to their all mountain capabilities. The key is stifness.
Thanks for the tips. What is a "mis stiff deck"? Where can I find the stiffness rating, what is a good one, and what is the range of the stiffness scale? Thanks. Have you had much experience with SIMS?