Insanely difficult games are the best unless it's in that crappy part of the first person shooter genre where the enemies have no AI, take more damage than the player in the supersuit, have bigger guns, and the only difference between difficulty levels is the amount of damage they deal when just standing there while you have no cover system and can only shoot them with half the accuracy they have. Penumbra was difficult in a good way and Unknown should stay like that.
Only hard problem solving, but there should be some challenging areas though. I heard there wouldn't be a lot of enemy encounters in Unknown, but I still think chase sequences and such should be mixed in to provide some difficulty. Afterall it won't be a total puzzle game.