Throughout the 19th century, well-to-do Americans would buy furniture from the finest craftsmen in Europe and have it shipped to their New World estates. For those who lived East of Chicago this represented a symbol of their wealth and prosperity. Unfortunately, when the wealthy of the West tried to do the same thing they ended up with furniture that split from checking, misfit pannels, non-functional doors and drawers, and tennon joints that failed to hold the furniture together. Mind you, these items were made by the most notable craftsmen who were at the top of their craft and skill. They just had no comprehension of what a 5% relative humidity environment would do to the materials they were using. Living in Ireland, it is possible that you may have a problem with this perspective as well.
The European craftsman responsible for the furniture you describe we're entirely aware of the properties of timber and how timber behaves in service. The problems you describe occurred and still do for different reasons, mainly that of air conditioning and central heating and peoples lack of awarenesses of the glue lines used in days gone by. They were also aware of the relative humidity of the regions in which they worked and accommodated it. The furniture was never designed to be shipped to wetter/dryer regions. They always built locally and repaired locally, if they were to have shipped their work they would have made the recipient aware of the possible problems as antique restorers do today.
I shared workshop space with one of this countries finest antique furniture restorers some years ago and learned a huge amount as a result. He also learned a good deal from my working practice. I saw first hand the effects of modern and historical mistreatment of furniture and also the exacting standards they used in their work. They were quite remarkable considering the glues and tools they had at their disposal. The timber was as it is today, same properties, same behaviour.
I don't live in Ireland by the way.
I know that when I work on a furniture project that has any complicated joints that the humidity of the wood must be between 8 and 12 percent - or the work does not survive climatic changes. I also know that I have to make special provision when I am joining laminates to solid woods or inlay materials. This also applies when joining dis-similar types of wood (Spruce behaves quite differently than oak) This probably falls more into engineering than craftsmanship, but it is a design reality that I have to deal with. I have a hard time believing that a good luthier would not be as aware of this issue as I am and have craft solutions available to address these issues.
Moisture in timber is not described as humidity it is described as moisture content.
I can assure you I work to a much tighter moisture content than you do. The problem is not the moisture content of the timber in and of it self, as once you get below 10% on most timbers you are left with what is described as "bound moisture" that moisture takes decades or longer to drop significantly. The "free moisture" or the water that is free to migrate from the cell structure of the wood is pretty much zero at that point and is the moisture that is lost as a result of seasoning. Timber at this point will readily accept and lose moisture and quickly depending on where it is stored and the environmental it is kept in. The only way to stop that is to finish the timber with a protective coating (partially successful) or to maintain the environment that the timber is going to be kept at in service. There is no way you can avoid this and it is this that we are talking about.
To your last point, I would say that modern instruments (those made in the last 50 years or so) that fail due to changes in the normal range of humidity ( 20 - 70 %) are poorly designed - regardless of how competent the craftsmanship was.
I see far more main brand manufacturers guitars in my shop for repair than bespoke instruments. I have never had a guitar of mine returned for material or workmanship defects as a result of humidity problems. I have built guitars that have been gigged all round the world. If there was a way to obviate the problems of humidity in acoustic guitar
design we would all be doing it. Trust me there isn't. You need to keep a careful eye on the humidity changes and react accordingly. Every instrument is different because every piece of timber is different. This is what we are talking about.
A guitar should be more about how it sounds than about how it is built, but I believe a true craftsman knows how to blend the engineering with the witchcraft. It would be interesting to see if there are specific models and brands that suffer from this design problem more than others (the reason I mentioned my old D-60).
There is no witchcraft involved in guitar building. A luthier who built guitars that didn't stand up to the demands of the modern world would soon be out of business. Those that do make a go of it are completely aware of the properties of the material they work with and how they behave. If you have a suggestion as to how to eliminate these issues I suggest you patent it fast because everyone from the boutique builder doing 10 guitars a year to Martin and Taylor would be after your secret.
There is no make or model that is more or less a problem than others. Martins are over built in many cases and Taylors are a triumph of slick manufacturing processes dressed up as improvements. Both suffer from humidity related problems as do other brands in equal number.
A good guitar maker knows both the structural and acoustic requirements of his instruments and stands or falls on the results.
I believe a good analysis in this area would prove my point. I hear Taylor owners complaining about this fairly regularly, but I don't recall hearing these complaints from a Gibson owner (or am I just real biased).
I see about a dozen Gibson acoustics a year for de-laminating, neck resets, splits, 14 fret hump etc. They are just as likely to suffer if not looked after properly
I'm pleased you have never had a humidity related problem with your guitars. Others do and that is not a reflection of either guitar design or poor craftsmanship or awareness of the issues. It is entirely down to material properties and the expectations we have of our instruments and the lifestyles we lead. Having said that it is entirely possible to manage the problems with a little care and attention.
About this Yamaha D-60. Where did you get it? I have never seen or heard of one? I'm interested in it's origins and influences as most early Yamaha closely followed existing designs. Any links or info the only D-60's I'm aware of from the big guys are Martin, Guild and Larrivee?