This solder is an alloy of lead and tin. It will eat silver
It will do that if you heat it to silver soldering temperatures, but
as long as you use it at temperatures it was intended for it works
just fine. I sometimes use plumber's solder for attaching a raised
silver cup or bowl to a foot if I want to do it without annealing it.
This would not be kosher if it was all silver, but I am usually
mixing metals anyway. The plumbers solder we get now is lead free,
being 95% tin and 5 % silver, more-or-less. Takes a bit more heat than
lead-tin, but still very low compared to silver or gold.
If you are sloppy with this solder and any bits are in contact with
material that you gold or silver solder later on it will, as you say,
eat right in and be one hell of a frustration.