It's not hard, not even traumatic for them. I might have gotten a claw in one of my stockings, as I forgot to take them off beforehand, but I'm undamaged.
Since I took over doing it when Nomad and Chesh left the country, I only make note of it so that I can look back and see when they're about due to be done again - though it's sparked an interesting conversation or two.
*thinks* Well, I suppose it's a little traumatic for them while it's happening, but it doesn't hurt them and once it's done only one cat of the three is skittery for more than a few minutes. She also sometimes runs away from petting that she asked for, or sudden movement, or unsudden movement - she's a jumpy one, and even so, she's over it in a half an hour.