It's written in TBB 2 that ancient European bows (Stellmoor, Holmegaard, and Neolithic Yew) made bows with the back from the center of the tree (edge grain).
Comstock writes, "bows were made so the cut-through rings appear mainly as straight lines on the bow's back, parallel with the limbs. Also the bowyers carefully followed the radial grain of the wood from one end of the bow to the other."
I would think "following the radial grain" would be the most challenging part.