Will a lead-free soft solder used in such a way eat into metal or
be prone to breakage when used in this manner?
Yup. The corrosive effect is seen when the soft solder is elevated
in temperature beyond its normal flowing temperature. I've seen this
myself back when I was learning (self-taught) and didn't have a
wonderful resource like Orchid. It actually ate away at the sterling
silver. It's a tempting shortcut to take, but a dangerous one for the
longevity of your work.
The issue with the strength of the joint is that in true hard
soldering (known in some circles as brazing), the molecular lattice of
the metal opens up and the solder actually becomes part of the metal.
With soft soldering, the metal is not heated to the point that the
lattice opens up. The soft solder just sits on the surface of the
metal, much like a drop of Elmer's glue.
When that joint fails and the piece needs repair is when you'll have
the problem. Reheat (overheat the soft solder) and you'll see the
corrosiveness in effect. You can't just go to easy solder because
there's all the soft solder contaminating the metal. The only choice
is to repair again with soft solder (knowing you'll be doing it again
soon), or to try and abrade off all the soft solder and do it with
conventional silver solder. That's the harder but better way, but not
always possible, depending on the nature of the piece.
Wish I could paint a better picture, but as they say, "been there...
All the best,
Charlotte, NC (USA)