One advantage that a scratch-builder has over a kit manufacturer is that he can pick specific wood for each specific part. Kit makers don't usually have this luxury. They need to lay out parts based on an "overall best" for all the parts that are laid out on a single sheet. This kit chose a good-quality lightweight balsa for the ribs, which was a good choice I think. But the riblets, which are from same sheets, may have have benefited by being cut from harder balsa. The aileron LE piece and the TE of the aileron cut out were also of this same lightwieght balsa but in this case I felt that these parts needed to be a bit sturdier so I cut replacements (using the kit's parts as templates) from some much harder balsa that I had lying around. It's the same 3mm thickness and fits exactly the same, but makes that whole area a bit more solid. Again, the scatch-builder can afford the luxury of using whatever wood he wants, but kit makers often have to be more practical.