The POW can be interpreted many different ways; rarity is possible. If someone has the prerequisite items and wants to direct a GPU farm at mining a great item, I'd say let them. But other mechanics are possible.

Example: You craft ammo. You combine 2 items and then apply POW. The amount of ammo you get is equal to the POW (leading zeroes). This is asymmetrically beneficial, because each additional unit of ammo costs DOUBLE the previous unit, so it is better to find the most efficient sweet spot to maximize the resources you are sacrificing to create the ammo. (Each additional leading zero costs double the previous in POW terms). So you don't actually want the highest POW possible. 
 

 Damn this is really good. I want to try this. 

 Another idea that I had was to have the server give out a gametick bearer token, that is needed as an input when crafting items. The token is only valid up until a certain time, so you can only grind for so long. 

 A crafting window of sorts. 🤔
Could you randomize the window somehow in an equitible fashion or would a static value suffice? 

 👀