I know there are different species of hickory. But the ones I have dealt with are very difficult to split.
My first encounter was, I was building a log cabin in Georgia. I needed a Summerbeam, a 12 inch by 6 inch beam that is 36 feet long.
I decided to make it by hand like the pioneers did. I got the logging company to deliver to my job site two hickory logs, 18 inch diameter and 18 feet long. I was going to make the beam in two pieces, with a post in the center.
I got to work on that hickory with my axe and adze. I hit that one log for about a half hour and I didn't get far. It was almost impossible to chop down that hickory wood, it was like iron.
I just rolled those two logs over to the side, and left them there to rot in the woods. I called up my sawmill and had them saw me out two 12 x 6 timbers of yellow pine, and deliver them to my job site.
Several years after that, I got into a hickory tree that I was going to cut up and split for firewood, and I could not split a 20 inch hickory piece with my maul.
After that, I have just left the hickory splitting to someone else. I did order a truck load of cut and split hickory, and two years later when I was ready to burn it, the wood was just covered with sawdust from all the hickory bark beetles.
No more hickory for me.