That is great advice. Where you are talking about just allowing multiple attributes for a product catalogue you can also do 'EAV light' and have a 'user-defined

That is great advice. Where you are talking about just allowing multiple attributes for a product catalogue you can also do 'EAV light' and have a 'user-defined-attribute' table that goes Sku, attribute_id, value and another table with a simple list of attributes. I have seen this in production (not in Magento) for a product catalogue and performance was satisfactory, queries were fairly simple and maintenance was easy. I think I prefer your JSON solution but this keeps very much in line with standard relational thinking.