Sure, but for me, my game isn't about finding out if this NPC can resist torture. My game is about the players and what their characters do. So what does the NPC having this trait accomplish for the game?
Now, if they have somehow struck up a friendship with Giles, and he matters to them, that's great! But then me having a story where he's going to be tortured to death is seems pretty predetermined to me. I'd rather see what happens to him as a result of play, and then if it did involve capture and torture, to let the dice determine that. Again, otherwise, you've predetermined his storyarc, which seems an odd thing to do.
Such absolutes seem at the very least to block certain avenues of play. The guard who cannot be bribed... okay, that cuts off a possible way past this obstacle. Can we find another entrance and a different guard? Do we have time to wait for a shift change? Can we take him out without alerting anyone else? The trait shapes play. And that's fine... it should do so. But the more you do it, and the more you're determining ahead of time, the more you're defining how play must go.
Is that enouh for play to become a railroad? One instance of an NPC with a given trait? Likely not, no. But the prevalance of such in the wider campaign? That's a concern.
And for me, the absolute bewilderment on the part of sandbox advocates that this is something that should be considered is surprising.