Nickel is most certainly NOT harder than steel. You're thinking of chrome, which is also shiny like nickle. That myth gets repeated every time this subject comes up!
Saying the nickle on modern brass cases is "plated" is wrong, it is a coating put on by an electroless process. Nickle WAS plated on very early brass cases, the giveaway to ID those is they are only plated on the outside of the case. The beauty of the electroless process is it gets into every nook and cranny of the object being coated. Very desirable form complicated shapes such as our primer pockets and flash holes.
The hardness of the coating can also be controlled, being as how the coated brass will be used in guns, the hardness is made to work out to be softer than anything it will touch.
The initial reason for nickle plating/coating was for 38 special shells carried by LEO's in gun belt loops made of leather. Part of the tanning process had a chemical that ate brass, turning it into a colorful weakened state that also wouldn't chamber if the cop got into a gunfight, had to load from the belt.
I used some winchester .280 nickle coated brass for hunting loads. One shell got dropped in snow, wasn't found until the next summer. The case was still bright and shiny, but the copper bullet was black, as was the brass primer. It chambered and fired in a 7400 Remington. I seriously doubt a brass cased shell would have chambered, or would ever had been found in the first place.