Note that no test was done with parallel grain.
Grain was oriented 90º.
Dave
EDIT
I would like to see two other tests:
1. a parallel test
2. a scarf joint 1/8 (ratio) test
I think this is a pretty extreme test that shows the ultimate strength of the joint in a worse-case scenario. So in a best-case scenario - parallel grain, joint in shear - the results would be much higher strength, but...
1 - I think the results in a parallel test would be negligible, since the shear strength would likely be exponential with the length of the joint. The extreme cases would be the radially and flat laminated butts and shafts, and they're reinforced by butt caps, joint collars, and ferrules.
2 - The scarf joint is designed such that any compressive force is distributed, in the direction that the force is directed to. So the only strength that could possibly be measured is by pulling the joint apart, an unrealistic scenario as far as cues are concerned.
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One factor I think that matters more would be heat resistance, which PVA and some epoxies are probably not best at, like when a cue is left in a hot car. (I'm sure builders here would advise against it to their customers but they do it anyway.) Another issue would be "cold-creep", something that water-resistant PVA and some epoxies can be guilty of. Yet another would be glue hardness. Some glues may absorb (or decouple) vibrations and resonance, some glues may transfer them more. And yet another factor would be gap-filling ability. Some epoxies can fill large voids, PVA can fill some very small voids, while hot hide glue requires perfect joints (intolerant of voids).
I built a couple decent personal cues about 20 years ago when I had my cabinet shop. Right now my handiwork consists mainly of building acoustic and electric guitars. I do use manly type-I PVA glue. The best builders today insist on (fresh) hot hide glue, as it allows for the smallest possible glue line thickness for its strength, leading to (presumably) the best coupling of wood for sound transference. Now some of these joints are only 1/4" width, of softwood, glued on a bias, bracing a top only about 3/16" thickness, and withstanding about 180lbs. of string pull. The glue surface on the rim to the tops and backs are only 3/8" wide. PVA also dries very hard, and it's my main choice simply because of convenience. I do mix it up as to my needs - PVA type-I on most joints where glue up time is quick, epoxy when 1) I need extra clamp up time 2) I'm using oily woods like cocobolo 3) I don't want to introduce moisture to the joint (back seam, neck join to body), PVA type-II water resistant on parts I need to bend (and thus must introduce moisture), CA for inlays and capillary-action type gluing (some binding)...
The fact that many acoustics have survived nearly 150 years, untouched, is a testament to the strength of hot hide glue. Unfortunately it has drawbacks. It must be kept around 140deg. in a dedicated heating pot when in use. The parts, and room ideally, should be kept warm to avoid the glue from gelling and skinning over, and it does so very quickly. Being water-based does introduce the problem of moisture. There is no gap filling ability, so the joint must be 100% perfect to last. On the positive side, the fact that hot hide glue forces you to make a perfect fitting joint does ensure that what you make will be better, by virtue of better craftsmanship; it's very comforting to know that some slop or tolerance can be compensated for with epoxy (and filler). And the bond created with hot hide glue is tenacious; it will bond most anything to anything; it was once used to "etch" glass where a design was painted with hot hide glue on glass, left to cure, and chipped off, taking the smooth outer skin of the glass with it. It is also the only glue that is truly reversible, as hot steam would release the joint, and warm water will remove all traces of glue. With every other glue, it cannot be removed once absorbed by the wood, and thus wood has to be removed to re-glue, because most all other glues wont adhere to itself. In the acoustic guitar world, this usually means a devaluation of the instrument if it's of high value.