I've played the first two...and I'll tell you, they can be brutally difficult and complicated if you don't know what to do in the exact situation. And it's, in my honest opinion, a lot harder than most FPS games (even though it's an over-the-shoulder shooter) because you have to aim for much smaller targets, deal with enemies/traps/etc. that can automatically kill you, sometimes have to button-mash to save yourself from a gruesome and gory death, sometimes have limited supplies...and that's just Easy mode. It's fun, it's frightening, and the first two were genuinely a joy to play, but it can be freaking difficult as hell with the puzzles and the "dismemberment" system they have going - and from what Sum's saying, I don't know if he wants a game that's as difficult to play as the DS series.
On a mildly-related note, I completely disagree on the third game being anything close to great as a diehard Dead Space fan...but that is for another discussion.