Wow, thanks for writing this! I was also pretty disappointed about the lack of information about flexibilities, especially considering there are so many of them.
My wand is redwood and brittle. I have found only one other person on Pottermore with a redwood wand - and funnily enough that person's wand was also brittle; and we both also had the same wand core - dragon. Personally, I would like to statistics of the most common and rare wood/core combinations. I think that would be interesting.
I find your notes on brittle wands interesting, especially the 'unlucky' part. Redwood is like the felix felicis of wand woods, and selects owners who are "good at being lucky", apparently. Brittle and redwood seem to contradict each other in that sense, but perhaps the brittle aspect of unlucky is more of a perceptual thing of the owner (i.e. one may wonder why they are lucky), whereas redwood's lucky is genuine and objective? That is my impression of it, when I consider my track record with luck.
I'm inclined to agree that brittle wands would break easily with "explosive" magic, but I think this would be the case more-so for brittle wands in the wrong hands (like if the wand were stolen). I find it hard to believe any wand would break from using a spell alone, as wands are built to be conductive, not resistant to magic. That said, it might not favor 'explosive spells', although perhaps the dragon core could counter balance this weakness in the wand, instead of making it more prone to breaking (i.e. the 'heart' holding the skin together).
Other than that, wonderful job with the flexibilities!