Alberto Ferrari wrote an interesting article about a strange behavior of AutoExist in normalized data models. I always say that a star schema is the best practice in Power Pivot and Tabular data modeling. The issue described by Alberto is another good reason to avoid snowflake schemas.
I think that an example is better than many words. Consider this simple measure working in a star schema where all product attributes (such as Category and Subcategory) are in the same denormalized DimProduct table:
SalesOfBikes := CALCULATE ( [Sales], DimProduct[Category] = “Bikes” )
If you have a snowflake schema with DimProduct, DimProductSubcategory and DimProductCategory tables, you have to write a much longer and complex DAX formula in order to obtain the same result:
DimProductCategory[EnglishProductCategoryName] = “Bikes”,
ALL ( DimProductSubcategory ),
ISFILTERED ( DimProductSubcategory[EnglishProductSubcategoryName] ),
VALUES ( DimProductSubcategory ),
Which seems crazy, and actually it is…
The reasons are interesting and well described in the AutoExist and Normalization article on SQLBI.