I don't follow your argument about durability, unless you mean these are boutique frames designed to be hung on the wall rather than ridden. Any bike that is really made to be ridden is going to have the durability issue I refer to, namely: when you hit it against a brick wall hard enough, where does it bend? I will admit to not having Kvale's expertise, but seriously, do you mean to tell me a lesser frame is going to fail at the seat lug? No, it is not. The place any frame will fail is going to be at the head lugs as I described before.
I am not in the least disparaging Kvale's artistry, workmanship, expertise, etc. My point is the fancy workmanship around the seat cluster is not functional. It is artistic.
For an experiment, try holding a cardboard tube, like from a poster, in your hands. Put one hand at each end, and using only your hands, try to bend it in the middle. You'll find you can put a temporary bow in it (much like you see the top tube of a frame doing when you apply the Park frame straightener tool), or you can bend over either end, but (unless you push it with your knee) you can't bend it in the middle.
Next experiment, try butting the cardboard tube by wrapping tape around it. With enough tape you can strengthen the ends enough that you can put a more serious temporary bow into the middle, but if you bend it hard enough for the tube to deflect, it will still deflect at the end, or at the end of the butt.
If you make a smooth transition from the reinforced area to the thin area, you allow a greater portion of the tube to flex without deflecting, thus increasing the amount of flexing force required to make the tube deflect. I'm sure that's the theory, anyway. In practice there will always be a point at which the tube will deflect somewhere, and at that point it will fail. Good workmanship can, at best, increase the amount of force required to make the tube fail.
Well said.
There is a lot that goes into engineering/ building a custom frame. As a guy who appreciates both finely crafted frames as well as purpose built frames, I should mention that the frames are basically products of the material limitations.
For me the ride quality of a conventional lugged frame/fork is the best available. I don't think a simple mixture of materials in certain ratios can produce the same result in each application. I also don't think that ride quality has much to do with effectiveness or efficiency. I like riding steel bikes but as a builder other materials, gussets, fillet joints represent new frontier with new potential results. Not everyone wants a nice bike.
I have a custom-built Charles Roberts frame. I honestly think this bike is truly amazing. I think custom bikes are better if done properly.