At Trek in the '70s, we used either silver or nickel silver. Never on stainless though. Tbh, I never knew nickel silver took more heat until I started building again 15 years ago. I don't think the extra heat is apparent unless you are working with stainless. It's within the upper range of normal brass flux, which is why Paul Brodie can use his gasfluxer. Stainless seems to have a property where it's easy to overheat. Maybe it's easier to notice just because it's shiny, IDK. But unlike more normal steels, it gets a cruft on it when that happens that makes it hard to flow out filler. I didn't see that behavior when using the Cycle Design flux, but that also helped the filler flow out more easily at a lower temperature, so that also probably helped.

When I worked at Trek, I developed symptoms of what the internet now tells me is a sensitivity to nickel silver. So I don't use it unless I'm feeling cheap and working with stainless. You can go through a lot of fillet pro ($) making lawn sculptures. I also wear a mask now because of Mr. Bulgier.

On the subject of gasfluxers, doesn't the same flux work with brass and silver? That would be nice. I have a drawer full of flux jars.