It is CrMo says hardened steel. I wouldn’t bother upgrading a club uni to ISIS since it is an entry level cycle and with ISIS hub and cranks it will still be entry level with ISIS. Personally I would keep the club as a freestyle or leaner spare unicycle and buy an ISIS one seperately- hopefully before bending your soft steel cranks.
So in your opinion Roger, why are the UDC hubs not weak like the cheap unicycles, if they are not made of Chromoly? What magical material are they made of that is not hardened steel that is harder than steel and is cheap like steel? The good news is the hubs are relatively strong!
One step hops are fine. You can bend the cranks by jumping down a few steps and landing badly- with correct technique it may not bend but I would advise caution and sticking with one or two steps at a time to prolong the life of it. Hopping stairs is a useful skill that you shouldn’t miss out on.
If you think you may get into a lot of hopping it could be worth buying ISIS to begin with, depending on your cash pile and how soon you would want to upgrade. Everyone is different- I think you can never have too many unicycles so get both!