Where trunk wood meets root-flare the wood fibers are tough, and whenever the corners of my hinge was over a root-flare I've had a lot of wood pulled out of the trees. Pretty consistently with a lot of species in my parts. I've had root-flare rip the whole side of some Douglas fir out. Ruined some butt logs terribly. However bad that may be at times for commercial logs, over the years I've purposely cornered my hinge over root-flare to use the effect to help pull some trees around to a lay. There's a lot of fact in this.
Now just using root-flare by itself to guide a tree another thing. But when root-flare is incorporated with the corner of a hinge, or both corners!!,, it's going to be a strong hinge. That's my experience and theory about root-flare.
Wood fiber characteristics vary wildly, as Bob pointed out. And much like him I believe the strength of wood fiber is due mostly to the environment the tree grows in. A huge data base could result from any thorough research done on the subject. So much for science. In the meantime, at work, we rely on our experience and gut feeling when utilizing wood fiber to guide a tree. In any manner.