Generic revit RFA duct let’s you do liner (inside duct) and wrap (outside duct) insulation on the same piece of duct. With ITMs/Fab parts it only lets you do one or the other and not both.
I know it’s rare having both on a single piece of duct, but it’s a pain when it happens. And if you can do it with generic revit RFA duct then you should be able to do it with an ITM fab part duct.