Something that irked me in Oblivion (and Daggerfall, and morrowind to some extent.. really every game) is that all the bows are either made of metal or volcanic glass (glass, ebony) it doesn't really make any sense, and I think a variety of wooden/bone bows would be cooler.
For instance, your basic "beginner bow" could be made of yew, elk bone with tendon string, whereas the higher echelons could be engraved Mammoth Ivory bows or Argonian Black-Wood bows and stuff of that sort. Daedric bows could just be made of Harrada or some sort of deformed tree that grows on the surface of Oblivion, either that or just a really high quality bow with a daedra bound to it.
It just didn't make any sense that bows were made of metal and glass just so they could match the other weapons, IMO wooden bows would be a lot better and more creative.
Awesome idea! I don't think they should be entirely made of wood though, but at least the functional parts. It would be weird to go from glass and ebony in the previous games to all wooden bows. Maybe make the glass and ebony bows just have embellishments that make them more valuable and add some visual flair.
That being said, I believe you are correct in your theory that the intensive flexural modulus ratio, along with the modulus of elasticity of substances such as glass and ebony would create an extremely high weibull modulus that would effectively preclude them from being used in an application requiring such tensile strength. They did look kinda cool though...