My experience with burl has been varied. Amboyna, thuya, & manzanita have always been hard & strong for me. Sugar maple burl is hard. I recently have been working with some hickory burl that I believe is the hardest & strongest burl I have ever used. It's ebony like hard.
On the flip, I have used aspen, birch, big leaf maple, madrone, redwood, buckeye, etc. that were all incredibly soft & weak. Some of the big leaf maple & birch were strong & dense, but not all. In essence, the woods i'd expect to be hard were hard. Woods i'd expect to be soft were soft. Any burl I have used that was soft but should have been hard, turned out to be "punky", meaning half rotten.
Right now I have burl in redwood, buckeye, birch, aspen, big leaf maple, sugar maple, pig nut hickory, manzanita, black walnut, madrone, ironwood, mesquite, aligator juniper, & a few other domestics. In exotics I have Afghan olive, cocobolo, bubinga, east Indian roisewood, amboyna, thuya, teak, tambotie, afzelia, camphor, etc. I got lots of burls. All are suitable for cues if processed correctly. I core anything structural because of the grain orientation to the piece, which is most often irregular. Burls are fun. Always something different.