Well, all I can say is that both my Dad & I have used Hoppes on Nickel guns for 50+ years with no ill results - then one day ( a while back ) we read that the Hoppes will adversely effect the Nickel finish. My Dad passed last year and I did stop using it on Nickel, however I can honestly say that all the Nickel guns that were routinely cleaned with Hoppes are just fine and have had NO problems what-so-ever. I'm not saying the advise is wrong, just saying I have never witnessed any adverse effects of Hoppes on Nickel.
I just inherited all of his guns and one of them is a Nickel M36 (1st year production) and it has been cleaned with Hoppes a "zillion" times! This was his BUG and carried daily during his 30+ years of L/E and other than the usual carrying marks and light scratches, it still looks fine. Go figure!
Chief38