Stainless steel, aluminum, carbon steel, mild steel, copper, and a range of specialty alloys all TIG well. Stainless and aluminum are the two we run TIG on the most because they need the cleaner heat and the controllable bead.
TIG handles thin material that MIG would burn through and gives a controlled finish that stick welding can’t match. For mixed projects we’ll often TIG the visible side and MIG or stick the structural side.