The Harries technique is more stable than the FBI or neck index techniques because it gives some extra support to the shooting hand.
Personally I find that my accuracy is much improved using the Harries over the FBI or neck index methods.
When used properly the Harries also makes it very easy to index the light and the sights in the same place.
The positioning of the hands tends to make it fairly easy to get the indexes aligned right off the bat.
The Harries technique is fatiguing.
While the isometric tension used in the technique helps to stabilize your shooting platform, it also requires muscle tension, and that gets old real quick.
The positioning of the hands is also pretty un-natural and it takes a good deal of muscle energy to simply hold the Harries position even if you aren't using the isometric tension.
As a result you'll watch guys on the line start to drop the light and the weapon down to about a low-ready position and try to bring the light and the weapon up.
The Harries is more of a committed position than the FBI or neck index.
It requires slightly more time and effort to get out of so you can transition into the FBI or neck index techniques.