Copper was the metal of choice before stainless steel was available.
Stainless steel is about 18% chromium and 10% nickel.
So there's no problem with the elements in the cupronickel alloys.
Stainless steel is much more popular now because copper is so expensive.
Also, stainless steel is compatible with the caustics used for clean-in-place systems.
I couldn't find a price for cupronickel alloys online.
But since nickel is currently more than twice the price of copper, I'll guess that the copper nickel alloys cost a lot more than stainless steel.