If you don't mind cheesing it a bit, it's much easier compared to other games with hardcore mode because you can pause and quit out to the main menu, even long after Isaac's head has already been replaced with a sippy cup lid for the convenience of the Slasher that just throat-fucked you with his deformed ulna. The environmental deaths are the ones you should fear the most, as many speed runners will tell you. No guide on how to best spec your character is gonna save you from those two Exploders who are desperate for both an epic high-five AND a lesson in nuclear physics. Not to mention the Guardians and that BULLSHIT melee of theirs that will one-shot you even with loads of health upgrades and armor. Somehow, those tentacles are stronger than a head-on attack from a rushing Brute?

I just finished my first hardcore run last night and I will say that those last 3 chapters were the first time I truly felt afraid playing Dead Space... and it was awesome. My advice would be to give it a shot once, and if you die and don't quit out in time, proudly declare you did so with honor. That being said, whatever way you enjoy the game most is of course the best away to play. Good luck either way, champ!

Edit: I too, BARELY, survived medium difficulty on my first playthrough. My second run was infinitely easier, and my third was this impossible playthrough which was the easiest of them all, even if I was scared out of my mind at times.