Not going to lie, an arcane slot card was the last thing I expected from Nate Cho's deck. That being said, this is a strange card that probably has a better place in non-guardian decks, like maybe Akachi.
My problem with Flesh Ward is that for the same amount of resources, I could have just played Something Worth Fighting For which can not only soak horror in a big chunk, but can also do it for my allies and can soak it from Rotting Remains. Basically the floor on Flesh Ward is me taking a single whack of 3 horror from a non-attack, and either dying from it or assigning one of the horror to flesh ward, killing it without spending any charges.
On the other hand, the ceiling of it feels really unrealistic: I have to take four attacks from monsters that only deal one damage of one type, spread over four turns, and then I can take one of each from anything and kill Flesh Ward, mitigating a total of six damage/horror, which is pretty sick, but not at all realistic.
I think there's probably a use case for Flesh Ward as the thing to keep your sanity up while fighting monsters that do one damage AND one horror with their attacks, which there are plenty of in the game. You'll soak some attacks, and by the time Flesh Ward runs out your sanity and health are about even. Maybe you'll get lucky and completely cancel some attacks, and sometimes you'll eat two or more damage or horror and still get hit, but for 50% or 66% of normal, which is strong damage reduction.
The bottom line here is that if you can afford the investment and the time, Flesh Ward will outperform True Grit/Something Worth Fighting For. However, resources and time are in short supply in Arkham, and unless I'm playing Tommy I generally can't find the space for True Grit/Something Worth Fighting For, making me think the same will be true for Flesh Ward, except that because of how it works even Tommy doesn't want it. Maybe it is strong enough to justify its cost for someone like Roland, but I can't tell just from looking at it. I'm sure someone who can manipulate the cancel or the charges would like it a lot, but it looks pretty underwhelming for everyone else.
Update: played the basic Nathaniel deck and got this down very early. Initially, after taking a 2 horror treachery straight to the face, I was like, "I knew it!" But having it for basically the whole scenario did end up saving me 6 health/sanity. I think I end up liking it slightly more than SWFF or TG, but it is a bit selfish.