Here I suppose I should defend my choice of zither pins rather than the expected through the neck pins.  In  a word it
is because zither pins are better.  I've made a number of harps with taper pins and either type of pin holds the string in
tune quite well.
Folc harps have the pins set as low on the neck as possible for the harp's mechanical strength and for maximum hand
room for the player.  Drilling holes all the way through the neck and reaming them for the taper pins removes a great deal
of wood and therefore a great deal of the strength of the neck.  The pins act as splitting wedges against the grain of the
wood.  That's probably why the harps of antiquity had a metal band reinforcing the pin holes.
The objections to threaded pins one hears cited are that the player might bottom out the pin and shear the threads in the
wood thus making the pin loose and unable to hold the tension of the string.  And at any rate, it stands to reason, so the
thinking goes, that continually tuning and retuning the pin will eventually weaken it to where it won't hold the tension of
the string.  And yet in the various sources where I've heard these opinions expressed, I've never read of anyone
experimenting to see whether or not they are true.
As to the first, one need only drill the pin hole deep enough to prevent the pin bottoming out.  If the pin of a Folc harp is
tightened to the bottom of the pin hole, the string hole in the pin would have disappeared below the wood!  I think anyone
clever enough to play a harp can be trusted to not wind the sting into the very wood.
As to the second objection, just how much turning of the threaded pin would it take in order to loosen it in the wood?  To
test this, I drilled a hole in a scrap piece of black walnut, a common wood for harp making but also one of the softest tone
woods.  Over a period of weeks the pin was turned 360° a hundred or more times a day, far more use than pin would ever
receive in a lifetime of harp playing especially considering that to change a bronze string from natural to sharp typically
requires the pin to turned only one or two degrees.  After the pin had been turned in and out more than 3000 times, I drilled a
second hole and mounted a second pin to act as hitch pin and installed the heaviest gauge string (.036" bronze) between the
pins and brought the string up to about 60% of maximum tension.  The string is about 12½" long and so comes up to pitch
as D. For a number of days the pitch of the sting was checked several times a day.  After the pin had undergone 3000 full
rotations, it still stayed exactly on pitch.  Next I tuned the string to D# and then back to D natural several dozen times a day
for several more weeks.  This is more like what happens to real tuning pin, turned only a few degrees while under tension.  
After hundreds of tunings and retunings, and these following the abuse of thousands of times being torqued into the wood a
full 360°, the pin still held tuning exactly on pitch.
And small wonder.  Threaded pins are used to keep $60,000 Steinway pianos in tune where the tension of the string and
mechanical vibrations are far greater than on a folk harp.  My experiments would suggest that in the practical world, it is
simply not possible to wear out the tuning pin hole as a result of ordinary tuning and retuning of a harp string.
The standard zither pin is 5mm in diameter, for ease of calculation, let's call it
3/16".  The force trying to turn the pin in its hole is angular, expressed as force
times distance.  The longest, thickest string on the largest Folc harp is under a
tension of about 34 lbs, but again for sake of easy arithmetic, let's call it 32 lbs.  
This string wound around the tuning pin is exerting a torque of 32 lbs times the
radius of the pin, which is 3/32".  So the string under the most tension is trying
to loosen the tuning pin with a force of 3 inch-pounds.  That is, it's not very
much. The only task the tuning pin has is to grip the wood so that more force
than 3 inch-pounds are needed to turn it but not so tightly that it is difficult to
adjust.

In the experiment to the left is the same tuning pin that was abused by twisting
and grinding it into the wood far more than any living harp pin ever endures.  A
string is tied to the end tuning wrench handle, which is 5" long,  and a two-litre
soda jug full of water is attached.  The water weights about four and a half
pounds ("a pint's a pound the world around").  Let's call it four.   So the pin is
under more than 20 inch-pounds of torque, seven times the force exerted by the
strongest harp string,  and yet it doesn't move.

That is, it takes a great deal more force to turn a zither pin than is being exerted
by any harp string even when that pin has been rather sorely misused.  The
prospects of wearing out a pin by tuning it are practically nil.