I've been trying to make the NPCs' heads and bodies match. I'm using EVE HGEC and Robert's Male body replacers, and the heads really look out of place. First, the colour is slightly off. I've tried several facial texture replacers and it seems they deal with face only leaving the neck out. Secondly, the details on the neck are missing - the detailed normal mapped RM or EVE HGEC torso switches abruptly and visibly into a smoothed neck. And the border is clearly visible - that is called the seam, I presume.

Is this all inevitable or am I missing something?

Considering that you're replacing the body and head textures independently, and that the character generation combines a base texture with an age overlay and a color adjustment from the slider settings, the chances of not getting a seam are pretty small.

IIRC the only way it can become seamless is to have the age slider fully left (which I believe eliminates any contribution from the age texture). The head and body textures have to be made to go together, and you have to get lucky with the deviations from base color that you select. Since you can lighten/darken and change tint on parts of the face, you may get bleed as far as the neck seam from some of those, especially beard settings.

I usually see more of a seam at the wrists, and I have no idea which "face" adjustments end up affecting that area. I'd have expected the hands and body to get the same overall color shifts, but they frequently come out different shades or hues.

The skin tones are controlled by the EGT files. Luchaire once made a replacement for the Exnem/HGEC texture layout, though I don't know where to DL it nowadays.

EDIT: Found it. It's titled "Luchaires HGEC Body Seam Reducer". Can't direct-link due to the content of some of the images on that file.