"My uninformed guess would be that it's the untreated wood inside the body of the guitar, as well as the braces, that would be the most vulnerable. That's why I wonder about the wisdom of not putting a Humidipak inside the body of the guitar, as some prefer to do."
It's fair to assume the thinner pieces of wood will be the most reactive to moisture loss and gain. It would also be reasonable to assume different woods, though they have been "cured", will retain different amounts of oils which resist a change in moisture level. Rosewood is very dense and contains quite a few oils while, say, generic maple is, when plane cut, not as dense and is not filled with the same amount of oils. How the wood is cut will expose more or less end grain. Quartersawn wood, the type most often used for top and back woods, shows the grain pattern but does not expose excessive amounts of open grain to the environment. Quartersawn wood is the strongest wood across the grain for any species/variety which gives it the preferred characteristics for a musical instrument.
Think about putting a thin sheet of wood in an uncontrolled environment along with a 2X4 piece. Which is the most likely to react to the environment first? The 2X4 represents the neck material on your guitar.
The problem the accessories manufacturers were presented with was simple, how to humidify the neck? Lacking a good answer to that question, the next issue became how to sell a "guitar humidifier"? Oh! look! there's a hole in the top plate!
I would say you are best to realize your guitar is not one, big, monolithic thing. It is made up of parts and most of those parts are not the same wood variety as the rest. The woods are all cut to different thicknesses and the manner in which the pieces are cut may vary from piece to piece. Due to the way quartersawn uses the log, it is also the most wasteful of the entire log.
There are multiple reasons for damage to occur and there's simply no way to address each without somehow cheating the others.
Also, soundhole humidifiers begin by releasing a good amount of moisture into the interior of the guitar. Soon the sponge or other device begins to loose its moisture which means it is giving up less to the guitar. Unless you constantly monitor and refresh the soundhole humidifier, you are actually exposing the interior of the guitar to far greater extremes than had you simply not used the humidifier.
This is not an argument against soundhole humidifiers, it is simply pointing out the obvious. If you live in an extremely dry climate or where the HVAC system is removing a good amount of moisture from the room, then you might find they have their place. Just stay on top of the amount of moisture in their sponge. Consistency, not extremes, is the key to maintaining your instrument.