Back when I was getting started making guitars we moved into a house that had a large elm tree on the driveway which had been killed by Dutch Elm disease. I considered cutting the stump down and making some of it into guitar sets, but at that point it had been colonized by Plurotus ostryatus mushrooms, which are good to eat, and had probably themselves eaten away at the wood. We used to harvest those by the shopping bag full a couple of times a year. We did get a lot of elm when we scavenged fire wood for our stove. That was not much of a bargain. The old 'Fire Wood Song' has a line: "Elm wood burns like graveyard mould, e'en the very flames are cold", which is just about right. You had to throw a lot of that stuff into the stove to get anywhere. It was also a pain to harvest, since it's so interlocked as to be nearly impossible to split. We used to actually rip the larger logs into quarters with chain saws, producing lots of elm excelsior. Anyway, the long and short is that I never did use any as guitar wood, nor have I ever measured the stiffness and density. The few reports I've heard rate it as being a pretty decent tone wood, though.
Any wood can be 'sensitizing' if you work with it long enough. My violin making teacher became sensitive to both spruce and maple when she was in her 70s. Trees, of course, can't run away when something bites them, so they resort to chemical warfare. That's what all the 'extractives' are that give wood it's colors and odor. Tropical woods have lots of enemies, and lots of ways to fight them off. It would be surprising if some people were not sensitive to at least some of those. I find some samples of ebony to be extremely pungent: I may not be 'sensitive' (yet!) in the sense of breaking out in a rash when exposed, but it can make me sneeze nearly uncontrollably, which is not good when you're re-sawing the stuff. I had one student who was allergic to Indian rosewood and morado, to the point where he could not touch them.