I think brittle is a pretty good description for some woods. Mostly from a tension standpoint. Black Cherry, I would describe as brittle. The back can at some point just snap like a candy cane. Some woods can be heavy and yet lack elasticity. If it is lacking it in tension, then it can seem brittle. Live Oak, based on my limited experience with it. If lacking in compression, then it tends to take more set. Hackberry I would put in the last category, though it can be greatly improved compression wise by heat tempering.
I think the key is either starting with a wood that has a good balance between compression and tension elasticity, Osage as an example, or bringing those things into balance thru design and modification (tempering, trapping etc..).