I split by hand and the worst splitting wood for me has been sycamore. Stringy as all get out, But have not knowingly split any elm and I've heard that the worst. Hard to imagine though.
I pulled some elm out of a local river bottom about 2 months ago for a buddy. All I knew is that it was dead standing wood with no bark and dry. I went to split if for him and OMG!!!!! I spent probably 4 hours splitting about 10 chunks of wood! That crap WILL NOT SPLIT! You would have to pound a wedge all the way through it and still pry it apart!
My buddy loved the way it burned but I told him unless it's river ash, he can split it himself!