Not sure why walnut would fail ,,as said before,each board you cut lams from is different.
Bocote and cocobolo are very greasy,just try to get finish to stick to a riser made from them. I've made a bow with bocote lams,just wirebrushed and blew the lams off,it is still shooting in AK as far as I know.
The 3 bow failures I've had were
maple core(bad glue joint,loss of clamp pressure)
red elm(bad glue joint,glue starvation,epon-versamid,Binghams said red elm soaks that glue in and can starve)
red elm (super long draw,shorter bow??)
Sixby noted walnut burns easily,but purpleheart seems worse to me,and I go from 50 grit on most woods to 36 on bocote and coco to prevent belt clogging.
The test bow I'm workin on now may shed some lite tho with p-lams and tip wedges,it may work the mid limb harder...???
All just my observations,yer mileage may vary!
Merry Christmas guys!!