Adding BOM & Routing to SKU's was a mistake...
If you have 1000 items with 2 variants each, that is 2000 variants. if you then have 3 locations you potentially need to maintain 6000 SKU's.
Customers struggle to maintain SKU's already.
BOM & Routing should have been added to the Item/Variant table.
Along with the a Standard cost and the existing BOM & Cost Explosion functionality.