There are a lot of unsuspected things going on around the area of the join between the neck and body. Some makers regard the thicker parts of a guitar neck as being “unbendable”, but the bending forces in the neck near the body joint are higher than at any other part of the neck. The neck and heel are only made of wood, and will distort quite easily, even if it is much thicker at that point, so bending certainly does take place, and even a tiny movement at that point has a considerable effect on string height. Not many makers take this into account.
Most truss rods do not influence the angle of the neck to the body because they usually have nothing to anchor to beyond the neck joint, and slowly but surely, the angle between neck and heel will open out under string tension. This is one of the two main factors in “neck rotation” that might justify resetting the neck to a new angle after a period of time.
I would compare this to the old strong man trick of holding a dining chair up by the end of one leg- enormous leverage on one side, but very little to hold on to it with, and no-one can hold on for very long.
Something else that is not often noticed is that higher actions put a much higher stress on the upper parts of the neck. This is doubly unfortunate, as it means that once distortion starts, the stresses can become even greater.
Looking at an old Gibson mandolin will show the extra material that was left in the area where the neck blends into the heel shape, rather than the deep sharp “throat” of many newer designs. It’s a theory of mine that old instruments are well regarded partly because time has filtered out the bad ones into the rubbish bin, and Lloyd Loar mandolins have survived extraordinarily well. That extra wood looks lovely, but serves mainly to reduce the stress in the neck at the point where the bending forces are greatest.
Non- engineers might need to research the difference between stress and force to really get to grips with all this.
Another clue to the stresses involved comes when resetting a neck. Wood is a “ plastic” material, and the effects of years of string tension can be mimicked with a few minutes of heat and moisture- removing a dovetail neck with too much brute force and steam can easily change the angle between the fingerboard and the heel even before the angle is trimmed with chisels.
Distortion of the neck near to the body occurs in most acoustic guitars, and is the cause of many neck resets, but it isn’t often acknowledged and the blame is usually laid elsewhere. Unless something is done to strengthen the neck to heel angle, the timber near the heel will slowly distort, allowing the neck to hinge forwards, rotating around a point in the neck slightly above the body join, no matter what truss rod or neck joint has been used.
For some years I have been reinforcing the heel of my guitars with an aluminium bar that is also part of the neck joint, and I’ve noticed some other makers taking a similar course. As an improvement to that idea I now bond the heel insert to the truss rod itself, so that the angle of the heel is not subject to slow “creep” under load. I have a refinement in mind when I have time, but so far, so good.