For unreadable dated buffalo nickels, it is wonderful. It leaes a stain, but if they are unreadable dates they are in bad shape already, so you are not destroying any value. And it DOES bring the date out from nowhere, like magic.
I only use it on unreadable buffalos, and one bottle will last me until the year 2034, but I like the stuff for what it is made for. :-)
It works on the mintmark for Buff's also I love the stuff. When you find Buff's on eBay list as 'acid restored' those are Nic a Dates. Take a look and you'll see the results.
There's no risk in my mind. What's a dateless buffalo nickel worth? My coin shop has bags of them for $.20 each. Try it on one you don't care about as much.