DAX table creation depending on boolean evaluation

Instead of using an IF, you can use a UNION and FILTER like this:

Test Group Dim Lookup =
VAR GroupByCat =
    IF (
        DISTINCTCOUNT ( Item_Master[CATEGORY] )
            > DISTINCTCOUNT ( Item_Master[SUBCATEGORY] ),
        0,
        1
    )
VAR _Union_ =
    UNION (
        FILTER (
            SUMMARIZE (
                Item_Master,
                [CATEGORY],
                "Boole", 0,
                "Avg Unit L", AVERAGE ( Item_Master[UNIT L] ),
                "Avg Unit W", AVERAGE ( Item_Master[UNIT W] ),
                "Avg Unit H", AVERAGE ( Item_Master[UNIT H] )
            ),
            NOT ( ISBLANK ( [CATEGORY] ) )
        ),
        FILTER (
            SUMMARIZE (
                Item_Master,
                [SUBCATEGORY],
                "Boole", 1,
                "Avg Unit L", AVERAGE ( Item_Master[UNIT L] ),
                "Avg Unit W", AVERAGE ( Item_Master[UNIT W] ),
                "Avg Unit H", AVERAGE ( Item_Master[UNIT H] )
            ),
            NOT ( ISBLANK ( [SUBCATEGORY] ) )
        )
    )
RETURN
    FILTER ( _Union_, [Boole] = GroupByCat )

I didn't invent this technique myself, but I don't remember where I learned it.