I've reloaded plenty of nickel cases in pistol & rifle and never had a problem with their "hardness" or with flaking of the plating. But you do have to watch out for it. One good thing about it is that is doesn't tarnish like brass does - so I use it for my pistol defense loads which sit for awhile.
I reloaded a set of Win .308 nickel cases over a dozen times (until the primer pockets got too loose from wear) and they always worked well. I DID anneal those cases every 3rd loading and that probably helped to keep them from getting brittle.