Definition of hardness
1: the quality or state of being hard
2a: the cohesion of the particles on the surface of a mineral is determined by its capacity to scratch another or be itself scratched
— compare MOHS' SCALE
b: resistance of metal to indentation under a static load or to scratching
That's just the wrong adjective, IMO. It can be used differently, but it's confusing if you think about it. What is hard about it? Is it to get Bitcoin? Is it because of the price that's why it's harder to get? I think the best description is the one that has something to do with the cryptography being solved.