Once again, nickel can be used on electric or acoustic.
This is what you see inside a set of D'addario XL's electric guitar strings (the reg pack with strings individually wrapped):
Then you have manufacturers that decided to market to players that think there's a difference between electric and acoustic strings (outside of nylons, you nit - whoever you are), and wouldn't put a set of nickels on their acoustic because they are "electric strings":
Also, why in the world would you think a guitar with the same gauge strings (but nickel wound) would warp the neck over time? Also, warp is probably not the term you are looking for because that can not be rectified with a truss adjustment.
Bottom line is nickels sound different than bronze. I don't like the sound of nickels on acoustics, nor do I like those dental floss wound nickels either, but it is preference and nothing else.