replying to clare, Iggy wrote:
That's what I initially thought, until I tried it and broke it down. A
Vertical's seam is 100% backed and the full panel's entire perimeter edges are
as well. However, a Horizontal seam only has a 1-1/2" support every 14-1/2"s.
That's only about 10% of a seam that's backed. And, no-one does nor would
waste the time to properly put any back-blocking behind the seam, which should
be required.

I can kick and hammer a Vertical seam and nothing happens. But, with
Horizontal I can just moderately lean against it to deflect it open and
completely crack the seam open and loose. I've heard your point of better
seam-hiding before and frankly I've only rarely witnessed it. I use just a 10"
knife on my seams and then lightly sand with a block or pole sander's plate.
No gaps, totally flat and entirely invisible, always.

You might enjoy My List. It's here, at the end/bottom:
https://www.homeownershub.com/mainte...ll-318143-.htm