In our B2B space there are tons of custom business rules. For example, you place orders per manufacturer and each manufacturer has its own minimum and reorder amounts and reqs on an order, promotions, business rules, etc. This is for business that have been around 25-30 years.

We did an evaluation on several out of the box ecommerce solutions and none of them were able to meet the requirements that absolutely had to be there. Shopify flat out said no, they can't help us, etc.