This is my take on honyaki, take it for what it is worth.
Honyaki is basically overhardened monosteel.
Japanese shallow hardening steels have few alloys in them, so to improve edge holding, a honyaki is hardened to higher RC. The claim that it holds edge better than san mai can only be true if some carbon is lost to the low carbon cladding in san mai (less carbon=less wear resistance), but you would need to lose a lot of it, to have a significant difference. The difference in sharpening (if any) should not be that noticeable (if both are hardened to same RC), but over time, you will need to thin it, and that will be harder. Same applies to flattening - it will be much harder to flatten honyaki than a clad knife, as you will be removing metal from a larger surface.
Why yanagi and not not santoku? Most double-beveled honyaki knives are not hardened to 63-64RC. You would have many unhappy customers, when they find out that 1" tip (or more) can break off just by accidentally dropping a knife. So, I would be surprised if a budged honyaki gyuto (oil quenched) is over 60RC (of course there will be exceptions). An yanagi, especially an expensive honyaki yanagi, one is expected to take a greater care, so those can be hardened to 63-64.
Steel in 63-65 range is very brittle. I can break a blank in that range in several pieces with a single hammer blow.
So what is this fuss about honyaki? The answer is what makes honyaki so much more expensive (and valuable to some) is the difficulty to make it. To put is simply, there is a high rate of failure (fracture, warping, etc), even by experienced makers when quenching monosteel in water in that thin cross-secion. So, buying honayki, you are paying a failure premium.
M
PS: A knife in W2 steel quenched in Park 50 (in quenching speed approaches water) with a clay coat will show hamon that is similar in appearance that that on White steel. Is this a honyaki? Yes, if you go by Japanese definition, but to me it's a monosteel knife in a shallow hardening steel differentially heat treated. Simple and clear.