It is very true that nickel-plated brass is harder and it can be tough to size.
One reason for some to use nickel-plated brass is for quick identification...Two of my grandsons are using Savages that I put together for them one is a .243 and the other is a .260...I chose nickel plated for the .243....There is no way to chamber a 260 in a 243 but we don't want 243 bullets rattling down the 260 tube either!....And how would you like to get to your deer stand and find out you brought the wrong ammo...
Also I have a 6.5-06 in the safe alongside a couple of 25-06s...Again I fashion the 6.5-06 stuff from 25-06 brass so the logical thing for me to do was use nickel brass for the 6.5-06....
There are also two 257 Bob AIs in there as well along side one of my favorites, a 6.5-257 Bob AI...I usually use .257 Roberts +P brass for the .257s and the head stamp is close....But I have used 7x57, 25-06, .270, and 30-06 brass to form them so I never count on a head stamp.
The 6.5-257 Bob AIs are extruded from .270, 25-06 or 30-06 brass because I can end up with full neck length after fireforming....But nickel is really tough to squeeze and I ended up wrecking more than I could use so I stick with all brass when forming them....
I color code the heads with a sharpie but even then a mistake is possible at the bench
( a .257 bullet in a 6.5 barrel will really wreck your SD on a string - DON'T ASK WHY I KNOW THAT)...I don't bring that combination of rifles to the bench anymore.
If I could find some nickel plated +P brass for the 257 Bob AIs I would try it just for the identification aspect...If the OP or someone finds a good source clue me in too please.
That's my take on nickel brass.
Good Luck,
Randy