Peghead or sound-hole? I've been thinking about this and I really prefer the headstock access. My reasoning is that cutting the hole in the transverse bar weakens the brace where strength is of utmost importance. Every Martin needs a neck reset, it's just a matter of time. Is this true of Gibson's and other peghead access guitars? I feel like that brace should be taller than the plans I have if I'm going to put a hole in it.

My next guitar will probably have access in the peghead and be a bolt on neck.

I firmly believe this is a weak point and part of the reason for neck resets.

I mostly use rods installed for soundhole adjustment, but I don't drill through the transverse brace. The rod stops at the neck block, and is adjusted by reaching behind the brace using the short leg of a standard Allen wrench.
With the advent of Allen adjustment nuts, it is a mystery to me why most of those headstock truss rods still have hex nuts, which require a large cutout in the weakest part of the neck to clear a socket wrench. With a recessed Allen nut, the peghead access hole only needs to be large enough to accept the Allen wrench.
OTOH, repairing broken headstocks does add to my income, so why complain?

My ability to maintain strongly held beliefs despite overwhelming evidence that I'm wrong is something that I struggle with. Once I form an opinion, it is very hard for me to change it. Maybe all of us are just wired that way.

In this instance, I think you are drawing a broad conclusion based on a small sample size. One Yamaha doesn't tell you what is going on with most guitars.

On top of the things you have heard from some very knowledgeable people, here is another data point for you to consider: All Martins with adjustable truss rods have the adjustment happen through a hole in the UTB. Which is more likely: That Martin (a company that makes avoidance of warranty repairs a high priority) is building a significant weakness into their guitars; or that the few guitars you have noticed with upper bout sinking problems had more serious issues than a hole in the UTB? Food for thought.

Well, if you were to go with a headstock truss rod adjustment, I would recommend (as Freeman does) that you use a scarf joint at the headstock instead of a one piece neck. The short grain on an angled one piece headstock, combined with a hole for the truss rod, makes a Gibson style guitar pretty prone to broken headstocks. In contrast to what I said about Martin, Gibson really does build a known weakness into its guitars (unfortunately). The repair folks can provide plenty of data on this.