Most of the advantages of scandium were for the builder, not the rider. It made the aluminum less prone to damage by welding so the maker could weld faster, not heat treat, have less waste, etc. In some cases this allowed them to use thinner wall tubing so some of the scandium bikes are a little lighter.
Most scandium bikes are nice bikes because it was a higher end market they were after, not because there's anything nice about the material. The marketing for these bikes was insane though... "Scandium is used in the nose cones of Russian missiles, so it makes an awesome bike!"