For example, if there are 1000 possible attributes, only 5% are populated for any given row. If you have 1000 columns you are going to have a ridiculously wide and mostly empty table.

There’s a number of ways to deal with the issue, e.g. a model per category or an EAV database pattern (this is what platforms like Magento does). None are really ideal, but storing the product attributes as JSON works pretty well. Even more so when your database supports querying the JSON blob.

Relational schema are flexible and allow for an optimizer to figure out how best to fetch your data based on the data are querying and filtering for and stored statistics about your data set.

Document oriented storage is great when you don't need the optimizer because you already know how the data is written and read. This means you can bundle it up into small, single fetch documents. No statistics or optimizer necessary. This is great if you understand your use cases really well, and they never change (good luck with that) or you have a large distributed data set that would be tough on an analyzer.

More importantly, JSONFields can be used as a substitute for EAV pattern.