I do not understand the reasoning (stressed importance) of direct over hinge line for the control horn linkage.
I have read and understand the offset advantage when installing a pull-pull. (Ackerman and the different travel distance)

The H9 U Stick 120 kit provided very small C horns with very little horizonital offset, (30% of the base would have been overlapping the LE bevel/taper of the ailerons and elevators) I have since acquired two larger sets with more offset. The largest set ((Dubro 366 / DUBQ 1985) has the greatest offset but they would require mounting with the hinge at the front edge of vertical. About 1/8" off vertical of hinge.

Are all kits so cheap with their hardware?
Short of the sky falling, what are the down sides of a little offset?

If the pivot point of the control horn is not on the hinge line you will get differential, meaning you will get a different amount of surface deflection in one direction than the other with an equal amount of servo rotation each way. If it's only slightly off, not a big deal as you can set the surface throw each way with ATV on a computer radio.

Does the 'pulling' on the hinge have anything to do with it?

Jerry