Every time I use Shift to Smooth, I obliterate appendages to the point where I have to resculpt them. What am I doing wrong?

Try turning on "unified strength". By default, brushes have individual strengths so you could be using inflate at a strength of 0.1 then smooth by holding SHIFT and it's at full 1.0 strength. Unifying the strengths feels more intuitive for me. It's in Sculpt Mode's tool panel > Options Tab > Options section > Unified Settings.