Riser materials are almost insignificant. Its what is bending that matters. Osage, hickory, yew, white oak, and maple make nice backings. Osage, yew, white oak, ipe and maple make nice bellies. Things to consider on lam bows:
Perfect glue lines from end to end
Very flat slats that are actually able to be glued flat and clean to one another
No knots on the back at all, not even pins or thorns. Same goes with a single piece board as well.