I've had both good and bad nickel. Some of it flakes off, and I've had small flake particles cold weld to the nickel matrix than holds the carbide particles in the carbide sizing ring together and start scratching cases. Had to polish it out with abrasives to get the finish back. I've had other nickel that just gradually wore off and never flaked, and that caused no issues. Seems like more than one plating process has been used. But if you are using, say, Lee sizing dies, they aren't terribly expensive to replace. I'll just advise you to apply a spray lube to the cases (even though you aren't supposed to have to with carbide pistol dies), and see if that doesn't discourage flakes from sticking to the surface. If I'd thought of it at the time I still had a lot of nickel 357 Mag cases I reloaded, I'd have done that, too.